• TABLE OF CONTENTS
HIDE
 Front Cover
 Title Page
 Table of Contents
 Acknowledgement
 Introduction
 Homogeneity of the records,...
 General climatic features
 Vegetation and climate
 Atmospheric pressure
 Precipitation
 Temperature
 Humidity, sunshine, and clouds
 Storms
 Wind
 Visibility
 Climate, diseases, and health
 Climate and sugar cane in...
 The upper air and general circulation...
 Appendix A: Notes on the organization,...
 Appendix B: List of hurricanes...
 Tables
 Bibliography
 Back Cover














Group Title: Scientific survey of Porto Rico and the Virgin Islands
Title: Scientific survey of Porto Rico and the Virgin Islands /
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Permanent Link: http://ufdc.ufl.edu/UF00091487/00018
 Material Information
Title: Scientific survey of Porto Rico and the Virgin Islands /
Alternate Title: Scientific survey of Puerto Rico and the Virgin Islands
Physical Description: 19 v. : ill. ; 24 cm.
Language: English
Creator: New York Academy of Sciences
Jay I. Kislak Reference Collection (Library of Congress)
Publisher: The Academy,
The Academy
Place of Publication: New York N.Y
Publication Date: 1942
Frequency: completely irregular
 Subjects
Subject: Scientific expeditions -- Periodicals   ( lcsh )
Natural history -- Periodicals -- Puerto Rico   ( lcsh )
Natural history -- Periodicals -- Virgin Islands of the United States   ( lcsh )
Natuurlijke historie   ( gtt )
Geologie   ( gtt )
Expedities   ( gtt )
Genre: bibliography   ( marcgt )
Spatial Coverage: Puerto Rico
United States Virgin Islands
 Notes
Summary: Includes bibliographies.
Ownership: Provenance: Gift of Jay I. Kislak Foundation.
Statement of Responsibility: New York Academy of Sciences.
Dates or Sequential Designation: Vol. 1, pt. 1-
Dates or Sequential Designation: Ceased with vol. XIX, pt. 1.
General Note: Latest issue consulted: Vol. 18, pt. 4 (1952).
General Note: Kislak Ref. Collection: Vol. 18, pt. 2 (1941)-pt. 4 (1952).
 Record Information
Bibliographic ID: UF00091487
Volume ID: VID00018
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 01760019
lccn - 2002209050

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Table of Contents
    Front Cover
        Page i
        Page ii
    Title Page
        Page iii
        Page iv
    Table of Contents
        Page 1
        Page 2
        Page 3
        Page 4
    Acknowledgement
        Page 5
        Page 6
    Introduction
        Page 7
    Homogeneity of the records, Tabulations
        Page 8
    General climatic features
        Page 9
    Vegetation and climate
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
    Atmospheric pressure
        Page 15
        Page 16
        Page 17
        Page 18
    Precipitation
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
    Temperature
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
    Humidity, sunshine, and clouds
        Page 39
        Page 40
        Page 41
    Storms
        Page 42
    Wind
        Page 43
        Page 44
        Page 45
        Page 46
    Visibility
        Page 47
    Climate, diseases, and health
        Page 48
    Climate and sugar cane in St. Croix
        Page 49
        Page 50
        Page 51
        Page 52
    The upper air and general circulation in relation to the weather and climate
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
        Page 63
        Page 64
        Page 64a
        Page 64b
        Page 65
        Page 66
    Appendix A: Notes on the organization, instruction, and history of the official D.W.I. rainfall stations, 1877-1917
        Page 67
        Page 68
        Page 69
    Appendix B: List of hurricanes passing over or near the Virgin Islands
        Page 70
        Page 71
        Page 72
        Page 73
        Page 74
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
        Page 80
        Page 81
    Tables
        Page 82
        Page 83
        Page 84
        Page 84a
        Page 84b
        Page 85
        Page 86
        Page 87
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        Page 125
        Page 126
        Page 127
        Page 128
        Page 129
        Page 130
        Page 131
    Bibliography
        Page 132
        Page 133
        Page 134
        Page 135
        Page 136
        Page 137
        Page 138
        Page 139
        Page 140
    Back Cover
        Page 141
        Page 142
Full Text






























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PORTO RICO COMMITTEE


1942



CHAR.LES P. E:F:KN,'. Chairman
JOHN HENDLEY BARNHART Eur.,ICE THOMAS MINER
CHARLES AM. BREDER ROY \VALDO MINER
BARNUM BROWN WILBUR G. VALENTINE
HOWARD A. IEYERRi-,IFF HOLPCE \V'. STUNKARD















COPYRIGHT 1942
BY
THt NEw YoRK Ac6rr,[y or SCIENCrs








THE NEW YORK ACADEMY OF SCIENCES


SCIENTIFIC SURVEY

OF

PORTO RICO and the VIRGIN ISLANDS


VOLUME XIX-Part 1


Meteorology of the Virgin Islands


Robert G. Stone


NEW YORK
PUBLISHED BY THE ACADEMY



















Editor
WILBUR G. VALENTINE


Associate Editor
ROY WALDO MINER










SCIENTIFIC SURVEY OF PORTO RICO AND THE
VIRGIN ISLANDS
This natural history survey of Porto Rico and the Virgin Islands, conducted
by The New York Academy of Sciences, was established in 1913. Continu-
ous publication of the results of this survey is made possible through contri-
butions from the Department of Agriculture and Commerce of Porto Rico,
and the University of Porto Rico.








METEOROLOGY OF THE VIRGIN ISLANDS

BY ROBERT G. STONE*

CONTENTS
PAGE
ACKNOWLEDGMENTS ........................................................ 5
INTRODUCTION .............. ............................................. 7
HISTORY OF THE OBSERVATIONS ............... . .................. 7
HOMOGENEITY OF THE RECORDS ............ ........................ 8
TABULATIONS ........................................ ...................... 8
GENERAL CLIMATIC FEATURES .............. ............................. 9
VEGETATION AND CLIMATE ..................... ............................. 10
ATMOSPHERIC PRESSURE .................................................... 15
Pressure Variations ...................................................... 17
PRECIPITATION ....................... ...................................... 19
Accuracy of the Measurements ............................................ 19
General Distribution ..................................................... 20
Forests and Rainfall ...................... ................................ 21
Orographic Effects .................................................... 22
Year to Year V ariation ................................ ............... 24
Diurnal Variation ....................................................... 25
Intensity and Frequency .......................................... 26
Evaporation ........................ ..................................... 30
Thunderstorms, Squalls, and Hail .................................. 30
Chem istry of Rainwater ................................................ 31
Water Supply and Irrigation ............................................ 31
TEMPERATURE ............. ................................... 31
Geographic Controls ................................................... 31
Extremes: Cool Spells and Warm Spells ................................. 34
Character and Effects of Changes ........................................ 36
Daily Cycle ............................................................. 36
Vertical Temperature Gradient ........................................... 38
HUMIDITY, SUNSHINE, AND CLOUDS .. ................................. 39
Absolute Humidity ...................................................... 39
Relative Humidity ....................................................... 40
Sunshine and Radiation ............... .............................. 40
Cloudiness ................................. ............. ................ 41
STORMS ...................................... ............. ................ 42
W IND ................................................... ............. 43
Surface Wind Velocity and Direction .................................... 43
Upper Winds .................... ...................................... 46
Winds and Weather Types (after Palgrave) .............................. 46
V ISIBILITY ............ ........................... ..... ................ 47
Harvard University, Blue Hill Observatory, Milton, Massachusetts; and Department of Me-
teorology, College of Engineering, New York University.





2 SCIENTIFIC SURVEY OF PORTO RICO
-, PAGE
CLIMATE, DISEASES, AND HEALTH ................. .................. 48
CLIMATE AND SUGAR CANE IN ST. CROIX (AFTER SHAW) ...................... 49
THE UPPER AIR AND GENERAL CIRCULATION IN RELATION TO THE WEATHER AND
CLIMATE ........... .............................. ....... .. ............ 53
APPENDIX A. NOTES ON THE ORGANIZATION, INSTRUCTION, AND HISTORY OF THE
OFFICIAL D. W. I. RAINFALL STATIONS, 1877-1917 ........................ 67
A PPENDIX B .................... . ......... ...... ................. 70
LIST OF HURRICANES PASSING OVER OR NEAR THE VIRGIN ISLANDS .......... 70
ACCOUNTS OF SOME VIRGIN ISLANDS HURRICANES .....'..................... 70
A PPENDIX TABLES .... .................... ... ............................. 82
ANNOTATED BIBLIOGRAPHY ............ .... ..... .. ........ .............. 132

Text Tables
1. Summary of Danish Weather Records at Christiansted, St. Croix, 1875-1916 11
2. Climatological Summary, Charlotte Amalie, St. Thomas, 1917-1936 ....... 12
3. Sea-Level Mean Pressures at St. Thomas .............................. 18
4. Frequency of Monthly Rainfall Totals Greater than Specified Amounts, St.
Croix ......................... ................................... 22
5. Average Rainfall for Each 10-Year Period, 1852-1911, of St. Croix ........ 23
6. Night and Day Rainfall, Charlotte Amalie, 1888 ......................... 25
7. Rainfall Intensities Measured at Jolly Hill, St. Croix, 1940-1941 ........... 28
8. Rainfall Intensities Measured at Anna's Hope, St. Croix, 1940-1941 ....... 28
9. Percentages of Days with Specified Amounts of Rainfall, Christiansted, St.
Croix, 1852-1907 .......................... ...................... 29
10. Average and Extreme Numbers of Days with Rain, Christiansted, St. Croix,
1852-1907 ...................................................... 29
11. Chemistry of Rainfall at Anna's Hope, St. Croix, 1911-1915 .............. 32
12. Temperature and Relative Humidity, January 15-21, 1914, at Anna's Hope,
St. C roix .......................... ......... .. ........................ 35
13. Ste.-Claire Deville's Temperature Observations at St. Thomas during 1840 37
14. Mean Temperatures at Different Hours at Christiansted, St. Croix, 1913-
1915 ................ ..... ... ........ ...... ... ................ .. 38
15. Temperature Difference between Charlotte Amalie and Louisenhoj, St.
Thomas ............................. ......................... 38
16. M monthly M ean H humidity ....................... ..................... 39
17. Vapor Pressure, Relative Humidity, and Cloudiness at Christiansted for Dif-
ferent Hours of the Day, 1913-1915 .................................. 39
18. Average Monthly Wind Velocity at the Evaporation Pan at Experiment Sta-
tion, Anna's Hope, St. Croix, 1920-1938 .............................. 44
19. Percentage Frequency of Wind Directions at Christiansted, St. Croix, 1875-
1907 ..................... .. .. ............... .. ................ 46
20. Sugar-Cane Yield and Rainfall of St. Croix. 1862-1938 ................... 51
21. Departures of Rainfall and Sugar Yield in Extreme Years of St. Croix ..... 52
22. Frequency of Annual Rainfall Totals and Correlated Sugar Yields of St.
Croix .......................................................... ..52
23. Rainfall of Five Wettest and Five Driest Years of St. Croix, 1852-1914 .... 53

Appendix Tables
1. Register of Stations and Available Records ............................. 82
2. Synoptic Table of Annual Rainfall Totals, Virgin Islands, 1875-1939 ...... 84







STONE: METEOROLOGY OF THE VIRGIN ISLANDS 3
PAGE
3. Average Monthly Rainfall, St. Croix Stations, Danish Period, 1838-1917, and
St. Bernard's, Tortola, 1831-1833 .................................... 85
4. Average Monthly Rainfall Totals and Days with Rain, St. Thomas Stations 86
5. Average Monthly Rainfall Totals and Days with Rain, St. John Stations 98
6. Average Monthly and Extreme Rainfall Totals and Number of Days with
Rain, American Period 1917-1941; and Tortola, 1900-1935 .............. 110
7. Average Monthly Rainfall for Estates in the Sugar Area of St. Croix, 1921-
1930 ............................................................... 114
8. Water Loss by Evaporation from Standard Weather Bureau Pan, Experi-
ment Station, St. Croix ............................................. 115
9. Average Monthly Temperatures, Virgin Islands (Danish Records) and Tor-
tola ............................................................... 116
10. Average and Extreme Maximum and Minimum Temperatures ............. 117
11. Average Number of Clear and Cloudy Days, and Prevailing Wind Direc-
tions, at Virgin Islands Stations, 1920-1930 ........................... 120
12. Monthly Temperatures and Rainfall at Bourne Field, St. Thomas ......... 121
13. Relative Humidity, Bourne Field, St. Thomas .......................... 123
14. Visibility, Bourne Field, St. Thomas ................................... 124
15. Cloudiness, Bourne Field, St. Thomas ................................. 125
16. Sea Temperature, Cloud Motions, and Ceilings, Bourne Field, St. Thomas 126
17. Mean Upper-Air Temperatures at Bourne Field, St. Thomas, in oC......... 128
18. Mean Upper-Air Relative Humidities, St. Thomas ....................... 129
19. Equivalent-Potential Temperature (Oe) and Specific Humidity (q) from
St. Thomas.Airplane Soundings, 1937, and Mean Noon Vapor Pressure
(e) at San Juan, P. R........................................... 130
20. Mean Upper-Air Pressures in Millibars at St. Thomas ................... 131

Figures
1. Effects of Weather on Vegetation .................................... 16
2. Rainfall Map of St. Croix, 1921-1930 .................................. 21
3. Average Lapse Rates over St. Thomas, 1937-1938 ........................ 55
4. Upper-Air Sounding at St. Thomas, March 23, 1938 ...................... 56
5. Upper-Air Sounding at St. Thomas, November 5, 1940 ................... 56
6. Surface Pressure and Height of the Trade (Ts) Inversion, July-November
1938, at St. Thom as ............................................... .. 58
7. Surface Pressure and Upper-Air Temperature and Specific Humidity at
3000 Meters, July-August 1938, at St. Thomas ........................ 59
8. Scheme of Upper-Air Structure over the Virgin Islands .................. 64
9. Mean Relative Humidity in the Upper Air over the Virgin Islands, 1937-
1938 ................... ......... ............................... 64
10. Monthly Mean Rainfalls, Mean Rainfall Intensities, in Inches per Rain Day
and per Rain Hour, and Mean Maximum and Minimum Temperatures, at
Bourne Field, St. Thomas, 1937-1938 ................................ 64







ACKNOWLEDGMENTS
This part of the volume on meteorology was not envisaged by the late
Dr. Oliver L. Fassig, who restricted his projected volume to Porto Rican
meteorology alone. While the present author was in Porto Rico in 1939
on invitation of the School of Tropical Medicine, the opportunity arose to
extend the scope of the study to include the Virgin Islands. The appro-
priateness of this extension is obvious; but it would not have been possible
to offer such a comprehensive discussion had the writer not been able to
visit the U. S. Weather Bureau at San Juan and the U. S. Marine Corps
Aerological Station on St. Thomas.
We are happy to acknowledge the favors of many people in the islands,
who, in one way or another, aided the work on this part of the monograph.
Special thanks are due to Mr. Morris de Castro, Secretary to the Virgin
Islands Government, to Col. Francis P. Mulcahey, in command of the
U. S. Marine Corps base at St. Thomas in 1939, and to Mr. N. N. Nichols,
Director of the Experiment Station, St. Croix, for facilitating the acquisi-
tion of data and general information in the field; to Sgt. Michael Davido-
vic, U.S.M.C. Aerographer, formerly stationed at the Marine Corps base
at St. Thomas, who under authority from his superior officers was most
helpful in putting at my disposal the facilities and observations of the sta-
tion at Bourne Field; to Mr. R. W. Gray, official in charge, and to his as-
sistants, Messrs. Kronberg, Cintr6n, and Maldonado, of the U. S. Weather
Bureau, San Juan, for their help in copying and tabulating data from their
files; to my own assistants at the School of Tropical Medicine, Miss Maria
Ruisanchez and Mr. George del Toro, Jr., for their faithful and intelligent
handling of many tedious computations and tabulations; to Hon. W. C.
Roy, Agricultural Officer of the British Virgin Is., for a transcript of
weather observations at the Experiment Station, Tortola. Dr. George W.
Bachman, Director of the School of Tropical Medicine, San Juan, and
Prof. Gleason W. Kenrick of the University of Porto Rico, have made
available for this part of the work all the resources which were also pro-
vided for the Porto Rican section, acknowledgment for which is made in
detail in Part 3.
The Blue Hill Observatory of Harvard University, the U. S. Weather
Bureau, and the Department of Meteorology of New York University
likewise supported, amply and in many ways, the preparation of this part
of the monograph along with the other parts. I am particularly indebted to
Prof. Charles F. Brooks, Director of the Blue Hill Observatory, for hav-
ing first initiated me to the interesting task of completing Dr. Fassig's





SCIENTIFIC SURVEY OF PORTO RICO


project and for his continued support through many delays and difficulties.
He kindly read the manuscript of this part in its preliminary stages, mak-
ing many valuable suggestions. Through discussions with him some of the
difficult meteorological problems of the region were considerably clarified.
The author had the good fortune to collaborate on a study of Caribbean
Meteorology at New York University, in the course of which many ideas
of value for the present work were gleaned from discussions with his col-
leagues, Prof. G. E. Emmons, Mr. J. E. Miller, and Mr. R. N. Culnan.
Prof. Earl B. Shaw of Worcester (Mass.) State Teachers College has
generously allowed us to reproduce passages and illustrations from his
Doctoral Dissertation on the Virgin Islands (Clark University, 1931).
Mr. Harold Larson, in charge of the Territorial Section of the National
Archives in Washington, kindly searched the Virgin Islands records in
the Archives for climatological material; he discovered the large series of
hitherto unpublished Danish West Indies weather observations which
we have summarized and printed herewith, a most remarkable and valu,-
able find. We are also indebted to him for translating several articles from
the Danish. Mr. J. B. Kincer, Chief of the U. S. Weather Bureau's Divi-
sion of Climate and Crop Weather, provided facilities of his staff for com-
puting the averages from these data.
We are grateful to Mrs. H. W. Dooley, Mrs. G. W. Kenrick, Misses
Maria Velasquez, Rose Vickers, Edna Scofield, Leoni Bleston, Mr. Ralph
Burhoe, and Mr. B. Acevedo, for kindnesses and help in certain particu-
lars.












INTRODUCTION
The chapters in this section are in the form of topical notes designed to
assist in interpreting the weather observations summarized in the accom-
panying tables and in the literature (see annotated bibliography at end).
No attempt is made to give a complete or well-rounded analysis of all the
elements nor a synthesis of the general character of the climate, as it does
not differ sufficiently from the climate of Porto Rico which is described in
Part 3 of this volume. The meteorological background of Porto Rican cli-
mate also applies generally to that of the Virgin Islands, because the lati-
tude, wind system, and surrounding ocean surfaces are essentially the
same. The differences are due mainly to the smaller area and lesser eleva-
tions of the Virgin Islands.
From the standpoint of climate, geology, soils, and vegetation, the
islands of Vieques and Culebra, political dependents of Porto Rico, should
be considered as part of the Virgin Islands; but the climatic data for them
are given in the section on the meteorology of Porto Rico, in order to con-
form to the political alignment, for the convenience of those who will use
the observations.

HISTORY OF THE OBSERVATIONS
The history of weather observations in the Virgin Islands is rather in-
teresting. Excellent data were compiled during the Danish regime, from
about 1825 on (APPENDIX TABLE 1). Educated Europeans working in the
islands early took an interest in the climate and as a result there are extant
several series of regular observations by instruments from the period be-
fore the Danish Government officially organized the taking of observa-
tions in 1876-77. Some official Danish stations were still reporting in 1917,
but thereafter the United States Government gave no special attention or
encouragement to these or other weather observers, except at Charlotte
Amalie and Christiansted, until the U. S. Weather Bureau began to estab-
lish more "cooperating" stations in 1920. The Bureau increased the num-
ber of cooperating stations in 1938-39. Since about 1830 many of the sugar
estates on St. Croix and some on St. John and St. Thomas measured rain-
fall for their own interest. Some of these observations found their way
into print, but most of them were inaccessible until 1911 when the Agri-
cultural Experiment Station began to collect the current data and publish
them. Many of the early records are lost. though some are on file at the
Danish Meteorological Institute in Copenhagen. Finally, the observations
of travelers and visiting ships, though covering very brief periods, may




8 SCIENTIFIC SURVEY OF PORTO RICO

illustrate some important details of the weather. Numbers of these appear
in the literature, but as there is no bibliographic guide to them, they remain
mostly unknown to those who might use them.

HOMOGENEITY OF THE RECORDS
It must be emphasized that as most of the various weather records for
the Virgin Islands are not strictly comparable with one another because
they cover different periods and were made in different ways, great caution
must be used in drawing conclusions from them as to geographical distri-
bution of temperature and rainfall. The interpolation of missing observa-
tions for certain days or months in a broken series, by comparison with the
records of nearby stations, is seldom justified in this region, except pos-
sibly with temperatures, because the weather is marked by decided local
contrasts. Localities only half a mile apart at the same elevation and in the
same valley can have very different rainfall values; likewise showers are
usually so limited in area that within a day or month it is largely chance
that two neighboring places receive the same rainfall. These anomalies of
locality and of short periods tend to equalize over longer periods, so that
the total annual rainfalls of neighboring stations generally show a parallel
or definitely correlated course, however different their normal or average
precipitation may be.*
In the tables, the records of the Danish period are in most cases sepa-
rated from those of the American period, because the kinds of instruments
used, instructions followed (see APPENDIX A), and other'circumstances
affecting the official records were uniformly different. Unofficial observers
both in the Danish and American periods probably used a variety of in-
struments and conventions. In most cases these circumstances are un-
known, and therefore it is well to accept such records with reservations.
On the other hand some of these private records are undoubtedly more
accurate than some of the official ones.

TABULATIONS
In APPENDIX TABLE 1 is a list of the stations and their periods of records,
with elevations, elements recorded, and place of publication or file. In-
quiries in the Virgin Islands and in Denmark would probably turn up
much more of such essential information. The records of the Danish co-
lonial government of the islands and files of some of the island newspapers,
The parallelism of the mean rainfall of two neighboring stations is tested by the constancy of
the ratios, month by month or year by year, between the rainfalls of the two. The parallelism of the
mean temperatures at neighboring stations is tested by the constancy of the differences between
the means, month by month or year by year.




STONE: METEOROLOGY OF THE VIRGIN ISLANDS 9

now deposited in the National Archives in Washington, were examined
for weather records by Mr. Harold Larson in charge of the territories sec-
tion in the Archives. He discovered the manuscript reports of the official
Danish observations for St. Thomas and St. John from 1877 to 1917; com-
plete except for a few missing months. As these had never been published,
except from 1877 to 1888 in the "Set. Thomae Tidendc", an abstract was
made of them and means computed (APPENDIX TABLES 2 and 3). The offi-
cial observations from St. Croix apparently were mostly sent to Denmark
where some were published and the rest probably filed in the Danish Ar-
chives or the Danish Meteorological Institute. Their inaccessibility will
not be so keenly felt, because we have fairly extensive published records
for many St. Croix stations since 1911, as well as the Christiansted record
from 1875, and some other miscellaneous data. For the other islands, how-
ever, the Danish period supplies the chief material, though very recently
in the American regime considerable data are again being obtained.
Relatively much less material is available for the British Virgin Islands.
In fact, Schomburgk's paper of 1837 is still the best discussion of the cli-
mate. The modern records cover only a few decades at Tortola and Virgin
Gorda. The absence of plantation agriculture in the last hundred years no
doubt accounts for the lack of interest in keeping weather records there.
Tabular summaries of pressure, temperature, rainfall, humidity, evapo-
ration, sunshine, cloudiness, winds, visibility, and some upper-air condi-
tions are given in TEXT TABLES 1 to 23 and in APPENDIX TABLES 2 to 10.


GENERAL CLIMATIC FEATURES
The general climatic features do not differ from those in certain parts of
Porto Rico, e.g., Vieques, Guayama, Arecibo, and Isabela, but on the aver-
age the rainfall and relative humidity are less, the sunshine greater. In-
deed the traveler from Porto Rico or from other rainier parts of the Carib-
bean will be impressed by the prevailing moderate dryness or semi-aridity
of the Virgin Islands, which the vegetation reveals at every hand,
The islands are so small that the climate approaches more nearly that of
the surrounding ocean than in the case of Porto Rico. Some of the ex-
tremes of temperature and rain recorded in the latter island are probably
never attained in the Virgin Islands. Owing to their more easterly location,
the influence of continental cold-air outbreaks from higher latitudes is
weaker than in Porto Rico. In the winter half-year storms passing across
the Atlantic in higher latitudes cause the same constant heavy ground
swells from the north as are observed in the islands farther west (a fact
noted and correctly explained by Schomburgk in the 1830's).




SCIENTIFIC SURVEY OF PORTO RICO


Before introducing details about the variation of each of the weather
elements over the islands, it may assist the general reader as well as the
specialist to have a concise view of all the elements for one typical station
on St. Croix and one on St. Thomas. The official Danish observations at
Christiansted from 1875 to 1916 are the most complete for any Virgin
Islands station and serve this purpose well. The averages of each element
are tabulated together in TEXT TABLE 1 for ready comparison. Further de-
tails for Christiansted are given in other tables under special headings and
in the appendix tables. The summary for Charlotte Amalie (TEXT TABLE
2) is a copy of the table prepared by the U. S. Weather Bureau for use in
the publications of the U. S. Hydrographic Office. The figures in these ta-
bles do not agree in detail with some of those given for the same place else-
where in this monograph, owing to differences in period of years covered,
or in the source of the data. Also, the wind-velocity and cloud-amount data
for Charlotte Amalie cover such a few years that they are probably mis-
leading as to details of the month to month variations.


VEGETATION AND CLIMATE
It has long been recognized that types of virgin vegetation as well as
various species of plants are usually limited in their geographical distribu-
tion by some climatic factors. In earlier times this obvious fact was often
used to judge the suitability of new lands for crops and settlers. Early
travelers to the Virgin Islands described its vegetation. Later observers
compared the contemporary scene with past accounts. In this way some
curious notions about the climate, particularly as to alleged changes with
time, have become popular in the islands.
The results of modern botanical and geographical studies do not reveal
any simple correlation of climate and vegetation. But when proper allow-
ance is made for non-climatic factors, such as soil, plant succession, effects
of deforestation, grazing, and cultivation, introduction of new species, etc.,
a general association of the boundaries of the "climax vegetation" types
with certain mean temperature and rainfall limits is evident.* "Climax
vegetation" is the ecologist's term for the virgin growth which represents
the most luxuriant that the given climate will support, the final equilib-
rium stage of the plant succession.
In the absence of extensive systematic instrumental observations of the
climatic elements, the climax vegetation may serve as a rough indication of
the climatic type. It is, however, a problem for the well-trained and ex-
Well described in James, P. E. "An outline of geography", N.Y., 1937; and in "Climate and
Man", 99-127. U. S. Dept. Agric.. Yearbook for 1941.















TEXT TABLE 1

SUMMARY OF DANISH WEATHER RECORDS AT CHRISTIANSTED, ST. CROIX, 1875-1916*


Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual

Mean max. temp. (17 yrs.,
1900-16) 'F. 80.8 80.8 81.7 82.8 84.9 85.6 85.8 86.2 86.5 85.8 84.2 82.0 83.9
Mean min. temp. (17 yrs.) F. 72.3 71.8 72.1 73.6 75.6 76.6 77.0 77.5 77.0 76.3 75.2 73.6 74.9
Mean temp. (17 yrs.) F. 76.6 76.3 76.9 78.2 80.2 81.1 81.4 81.8 81.8 81.0 79.7 77.8 79.3
Mean temp. (28 yrs., 1892-1916) F. 76.6 76.3 76.8 78.3 79.9 81.0 81.5 82.0 81.7 80.8 79.3 77.5 79.3
Highest temp. (33 yrs., 1882-1915) F. 86 87 91 91 95 96 94 91 91 91 92 91 96
Lowest temp. (33 yrs.) F. 65 66 64 67 69 69 70 70 69 69 69 65 64
Relative humidity:
mean (8 A.M. + 2 P.M. + 9 P.M.)
(17 yrs.) % 74 73 72 72 74 76 74 75 77 78 78 77 75
mean (2 P.M.) (4 yrs., 1913-16) % 6S 70 65 68 72 69 69 69 70 72 75 72 70
Cloudiness: mean (8 A.M. + 2 P.M. +
9 P.M.) (19 yrs.) tenths of sky 3.7 3.5 3.5 3.7 4.0 4.4 4.3 3,9 4.0 3.9 3.7 3.7 3.9
Rainfall: mean (37 yrs., 1875-1916) inches 2.32 2.05 1.24 2.97 4.39 4.53 3.46 4.26 5.59 5.88 5.89 3.87 46.43
Ave. no. days with rain (37 yrs.) 13 10 7 8 11 11 12 12 12 13 14 13 136
Ave. no. of thunderstorms (19 yrs.) 0.2 0.05 0.05 0.4 2 2 2 3 2 3 1 1 16
Prevailing wind dir. (to nearest
8 points) (4 yrs.) E E E E E E E E E E E E E

Reed, W. W. U. S. Mon. Wea. Rev. pp. 133-160. April 1926. Based on Dr. Neumann's records published in the Yearbooks of the Danish Meteorological Institute,
Copenhagen, 1875-1916. They were taken in the town proper near sea level, and with standard instruments. The instructions of the Danish Meteorological Institute were
followed. There were no observations for 1878-81, and those for 1876, 1877, 1899, and 1916 were not complete for all months.










TEXT TABLE 2

CLIMATOLOGICAL SUMMARY, CHARLOTTE AMALIE, ST. THOMAS, 1917-1936*

(Prepared by U. S. Weather Bureau)


Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year

Air temperature 'F.
Mean Mean
Monthly, 1917-36 77 77 78 79 80 82 82 83 83 82 81 79 80
Maximum, 1921-36 82 81 82 83 85 86 87 88 88 87 85 83 85
Minimum, 1921-36 73 72 73 75 76 77 78 78 78 77 76 74 76
Extreme Extreme
Maximum, 1923-38 85 85 87 89 89 90 90 91 92 91 91 89 92
Minimum, 1923-38 65 63 67 68 66 70 71 70 68 70 68 69 63
Rainfall Total
Ave. amount in inches 2.99 2.21 2.21 2.09 4.60 2.91 3.68 4.42 6.17 6.12 6.25 3.10 46.75
Ave. no. of rainy days, 1921-36 17.9 12.2 13.2 11.2 14.1 14.6 16.9 17.6 17.0 15.8 17.2 17.6 186
Extreme
Maximum in 24 hours in inches, 1921-36 2.33 1.56 1.50 2.81 7.28 3.41 5.28 4.65 10.02 4.15 3.55 2.12 10.02
Wind
- Mean velocity mi./hr. Mean
at 8 A.M., 1917-21 6.7 6.3 6.9 5.6 6.8 6.5 6.8 7.4 6.2 5.2 5.0 6.3 6.3
Mean velocity mi./hr.
at 8 P.M., 1917-21 7.3 8.9 8.1 5.8 6.1 5.2
Percentage of observations from Mean
N 1 2 1 3 3 3 t 1 $ 3 1 1
NE 30 24 17 11 5 2 9 14 13 15 24 28 16
E 56 55 58 58 55 67 78 64 51 52 42 60 58
(1917- SE 5 9 14 24 34 28 12 14 22 20 14 5 18
1936) S 1 1 3 3 4 1 t 4 5 4 2 1 2
SW 1 2 2 0 1 t$ 1 2 3 3 t 1
W t o t o0 t 0o t t 3 1 o t
NW t t 1 0 t 0 0 3 0 2 3 t
Calm 6 7 3 3 2 t : 1 5 5 9 5 4
Barometric pressure, in. at sea level, 1919-28 Mean
Mean (8 A.M. + 8 P.M.)/2 30.03 30.03 30.05 30.03 30.00 30.02 30.04 30.01 29.95 29.94 29.95 30.00 30.00
Highest 30.18 30.18 30.18 30.17 30.13 30.16 30.16 30.15 30.08 30.06 30.10 30.16 30.18
Lowest 29.87 29.84 29.85 29.88 29.83 29.91 29.85 29.73 29.30 29.71 29.75 29.81 29.30
Cloud amount (0-10), 1917-20 Mean
8 A.M. 4.2 4.0 4.0 4.0 5.3 6.0 5.3 3.8 4.4 4.3 4.8 4.0 4.5
8 P.M. 4.6 4.8 3.3 3.8 3.7 4.1

U. S. Customs House; two-story stone building in town on shore of harbor; elevation 27 feet; Lat. 18 13' N.; Long. 64* 29' W.
t (8 A.M. + 8 p..m.)/2 for June-November; other months 8 A.M. only.
3 A few cases, but less than 1 per cent.





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 13

perienced ecologist or geographer; the amateur is apt to overlook impor-
tant factors and thus come to unscientific conclusions.
Even the existing "secondary" vegetation, greatly altered as it often is
by the artificial influences of human settlement, may suggest climatic con-
ditions which are otherwise unnoticed or outside the scope of ordinary in-
strumental observations of the weather elements. This is particularly use-
ful in studying local influences of the vegetation on the microclimates, the
small-scale variations in the air layers near the ground which have much
practical importance for man and agriculture.
A description of the vegetation (not the flora) is therefore not out of
place here. Since practically no detailed quantitative ecological field studies
have been made in the West Indies, the interpretation of the vegetation-
climate relationships is necessarily conservative and general.
Borgeson, writing in 1898, sketched the forest situation succinctly:

"In consequence of their location, the islands have a tropical vegetation, but the
comparatively scanty rainfall does not, as a rule, permit the development of as rich
and luxuriant a plant life as is found in other places in the Tropics where the precipi-
tation is greater. In spite of this, however, the vegetation may be almost as rank as
that of the jungle in valleys possessing a sufficient supply of moisture. As in all
small islands, with a relatively large population, man's intervention has left its
stamp on the vegetation. The story goes that at the time of their discovery they were
covered with thick forests of which, however, hardly anything is left. During the
period of colonization the forests were burned off and the little that probably sur-
vived, along with what later grew up, has been partly destroyed by reckless cutting,
a practice which is indulged in up to the present time. There is nevertheless some
forest vegetation in the northwestern part of St. Croix, on some mountain slopes of
St. Thomas, and especially in St. John, but mostly of secondary growth. The origi-
nal vegetation in all probability is best preserved in the mangrove swamps along
the coasts and in the forests growing on the sandy beaches, where the quality of the
soil gives no encouragement to cultivation".

Although the vegetation was undoubtedly originally "forest" (i.e.,
merely arborescent) in most parts, it probably included some of the very
dry thorn-bush landscape which appears more widely today. St. Croix, St.
Thomas, and St. John, at least, were all deforested during the 18th and
early 19th centuries by cutting to provide timber and by burning to clear
for cultivation. On most of St. John the bush has been allowed to return,
and it is now returning to much of St. Thomas, although St. Croix, except
for the dry eastern end and the hilliest northern sections, remains largely
under cultivation for sugar cane or in pasturage for cattle. Grazing is also
an important industry on St. Thomas. Grass, thorn forest, and bush are
generally conspicuous on St. Croix today, which, owing to its greater range
of climates and soils, is said to have a greater variety of vegetation types
than the other islands.





14 SCIENTIFIC SURVEY OF PORTO RICO

There are numbers of endemic species, but they do not seem to have
specialized climatic adaptations. Britton and Wilson (1923: 4) observed
that

". .. they are scattered in distribution, not being restricted to wet, dry, high or low
districts or to specific types of soil, though many of them appear from our present
knowledge to be very local in distribution. All the endemic species are more or less
closely related to other species inhabiting other West Indian islands, indicating
community of origin, and differentiation through isolation".

The vegetation has been so altered by man that much caution must guard
any inference about climate from the present character and distribution of
the vegetation. The heaviest woods on St. Thomas are now growing in
certain ravines on the north side of the island, but one cannot conclude that
the rainfall is therefore more abundant in that vicinity; evaporation, to-
pography, desirability of the soil for other purposes, charcoal burning,
grazing, lumbering, and accidents of plantation abandonment also have to
be considered.
In connection with this question, the following statement in a recent
official publication* is of interest,

"It has been generally supposed that the Virgin Islands were once covered with
mahogany and other valuable cabinet woods and that dense forests of these trees
were cut off in the early days and then again later to make place for cane when the
slave labor cultivated even the steep hillsides. Qualified foresters, however, consider
this extremely doubtful, and do not believe that these cabinet woods were ever plen-
tiful on these islands. They do grow well here, but the specimens scattered sparsely
throughout the islands or standing in clumps about old ruins and along the sides of
many roads, are believed to have been imported. Little natural young growth of
these trees is to be found, but thousands are now being set out, and seem to do well.
Most of St. Thomas and St. John and much of St. Croix is thickly covered with
many different types of trees, but these are not of accepted commercial value, nor
of size or character to be considered forests. Where the land is not cleared for cul-
tivation or grazing, these scattered trees are interspersed with dense growths of
underbrush and vines that rapidly reclaim any land that is left uncared for even a
year or two. Many of these trees afford food in the form of wild fruits, such as the
mango, the soursop, etc.; but most of them are valuable only as wood for the burn-
ing of charcoal the universal fuel of the Virgin Islands".

The dry vegetation of eastern St. Croix, where the rainfall probably
averages but 25 inches or less a year, considerably resembles some of that
of the semi-arid southwestern United States and northern Mexico.t
The trees and bushes on exposed slopes have been forced by the prevail-
"The Virgin Islands of the United States", p. 5. U. S. Dept. of the Interior, Washington,
D. C. 1935.
Ct f. Shreve, F., Lowland vegetation of Sinaloa. Bull. Torrey Bot. Club 64: 605-613. Dec. 1937.
Britton, N. L., Cactus studies in the West Indies. Jour. N. Y. Bot. Garden 14: 99-109. 1913;
The vegetation of Anegada. Merm. N. Y. Bot. Garden 6: 565-580. 1916. Gleason, II. A. & Cook,
M. T., Plant ecology of Porto Rico. Sci. Surv. Porto Rico and V. I. 7: 1-173. 1927.




STONE: METEOROLOGY OF TIHE VIRGIN ISLANDS 15

ing trade winds to grow with a bend to the leeward; this asymmetry is
presumably due to the effect of the salt carried by the wind and deposited
on the exposed branches. The wind-deformed vegetation points rather uni-
formly towards west by south, suggesting that the prevailing wind direc-
tion is from slightly north of east. Actually such a wind direction predomi-
nates only for the winter season, so that probably the stronger winds and
more salt spray of the winter months have a predominating effect on the
vegetation (see FIGURE 1A).


ATMOSPHERIC PRESSURE
Barometric observations in the Danish period are available from Horn-
beck's record (1833-51) at Charlotte Amalie, from Ste.-Claire Deville's
(1848) and Stenzel's visits (1886) to St. Thomas, and from the official
Danish station at Christiansted (1875-1916). During the hurricane sea-
son (July to October), special daily barometer reports have been made at
St. Thomas since 1878, first by A. Wall6e and later by the harbor masters.
These were cabled or radioed to San Juan or to the United States for the
U. S. Government weather service. It appears that these observers read
the barometer in other months also, but only the means computed from
these reports for the years 1919-28 are available (TEXT TABLES 2 and 3).
Such reports were also made from Christiansted, but no tabulations were
published. For some years after 1911, the Experiment Station at Anna's
Hope, St. Croix, maintained an aneroid barograph, and photographs of
the daily charts for several years are reproduced in the first two reports
from the station. These give as adequate a picture of the pressure varia-
tions as necessary for most purposes, and their value is enhanced by the
concurrent temperature curves which are reproduced in the same reports.
The extremely low pressures experienced in hurricanes are illustrated in
Appendix B. Barograph records are now being kept at the aerological sta-
tion at Bourne Field, U. S. Marine Corps, on Lindbergh Bay, St. Thomas,
and probably at the new Army air bases on St. Croix. No pressure read-
ings for the British Virgin Islands are available except those of Schom-
burgk (1833-37), which are not reduced to sea level. All the early 19th
century observations are from instruments of doubtful accuracy.
As in Porto Rico, many planters, merchants, and professional men own
a barometer of some sort which is watched during the hurricane season for
signs of local disturbance, in spite of the efficient broadcast warning serv-
ice now available from the U. S. Weather Bureau at San Juan. These in-
struments are often improperly calibrated, uncompensated, and exposed










SCIENTIFIC SURVEY OF PORTO RICO


B

FIGURE 1. Effects of weather on vegetation.
A. Tree deformed by prevailing wind. Picture taken a few miles east of Christiansted, near
the north coast of St. Croix. (Photo by E. B. Shaw, 1930.)
B. Dry weather is sometimes fatal to the palm trees as well as to the sugar crop. These palms
"lost their heads" during a severe drought period on St. Croix. (Photo by E. B. Shaw, 1930.)





STONE: METEOROLOGY OF THE VIRGIN ISLANDS


at various elevations, so that the readings from them are usually reliable
only for showing relative changes in the pressure rather than the absolute
values.

Pressure Variations
The barometric pressure mean values and variations observed in the
Virgin Islands are entirely similar to those experienced at San Juan where
an extensive and accurate series of readings has been kept by the Weather
Bureau since 1899. The average is lowest in May and November and high-
est from January to March and July to August. The semi-diurnal daily
cycle of the barometer (i.e., double maximum-double minimum at about
10 A.M. and 11 P.M., 4 A.M., and 4:30 P.aI., respectively) is the most pro-
nounced feature, as everywhere in the tropics, and at San Juan has an
average amplitude of .063-.085 inch (= 2.14-2.98 millibars), largest in
February and July, least in May and November.
There are occasional spells of some days' or weeks' duration that may
occur in any season, when the regular daily cycle is largely suppressed or
greatly disturbed by various factors, including cloudy or rainy spells, hur-
ricanes, and longer-period pressure "waves". In addition there are gentle
swells in the general level of the barometer curve, rising and falling in long
steady swings lasting several days to several weeks. These are often asso-
ciated with noticeable changes in temperature and rainfall and are due to
disturbances of the trade winds set up by passing cyclones and anticyclones
in higher latitudes in winter, and in summer to the changes in the intensity
and position of the Azores-Bermuda anticyclone and passing allobaric
systems.* In the winter half of the year nearly all the coolest nights re-
corded at the Experiment Station at Anna's Hope, St. Croix, occurred
during the middle of a few-days period of relatively low pressure for that
season, and vice versa the warmest days (highest temperatures) come at
times of higher than normal pressure for the season. This relation is prob-
ably general, at least for all sheltered locations in the interior of an island.
Knox (1852) correctly observed, ". .. with NE winds the barometer
almost invariably rises, and generally falls when the wind is SE or S. We
have observed it to rise to 30.15 inches with a fresh NE breeze, but this
great tide seldom takes place. Passing showers seldom or never cause the
mercury (i.e., the barometer) to fall . ." The little fluctuations on the
barograms when showers or thunderstorms pass are usually less than one
millibar in amplitude.
Frolov, S., Bull. Am. Met. Soc. 22: 198-210. 1941; id., "On the synchronous variations af ft,
pressure in tropical regions", in the same journal, (5) 1942. Dunn, G., Bull. Am. Met. Soc. :tN, U I CA ,
215-229. 1940.





'47'

















TEXT TABLE 3

SEA-LEVEL MEAN PRESSURES AT ST. THOMAS (IN INCHES)*

(With Extensions by Reduction from Readings at San Juan, P. R., 1899-1933)


Hour, Atlantic Standard Time Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct, Nov. Dec.

8 A.M. (1919-28) 10-year mean .... .... .... .... .... 30.03 29.99 29.95 29.93 ........

6 P.M. (1919-28) 10-year mean .... .... .... .... .... .... 29.99 29.95 29.91 29.89 ........

8 A.M. + 8 P.M. 10-year
mean (1919--28) 30.03 30.03 30.05 30.03 30.00 30.02 30.04 30.01 29.95 29.94 29.95 30.00

5 Probable true 24-hr. mean 30.01 30.01 30.00 29.97 29.96 29.99 30.01 29.97 29.92 29.90 29.91 29.97

Reduced to 30-year normal
(1899-1933) (compared mb.* 1016.9 1016.9 1016.6 1015.6 1015.2 1016.3 1016.6 1015.6 1013.8 1013.2 1013.5 1015.6
to San Juan) in. 30.03 30.03 30.02 29.99 29.98 30.01 30.02 29.99 29.94 29.92 29.93 29.99

Probable highest reading (1899-
1933). Estimated from San Juan 30.23 30.22 30.23 30.20 30.17 30.16 30.19 30.18 30.12 30.19 30.14 30.18

Probable lowest reading (1899-
1933). Estimated from San Juan 29.76 29.70 29.77 29.71 29.73 29.82 29.60t 29.28t 28.83t 29.57t 29.65t 29.78

Since 1939 the U. S. Weather Bureau has been reporting pressures in millibars rather than inches; tables for conversion from inches to millibars or vice versa (and
from millimeters) will be found in the "Smithsonian Meteorological Tables" and other handbooks of meteorology. The factor is 1 inch = 33.8640 mb. The mean station
pressures at Bourne Field, 1937-38, are given in the table of upper-air data in APPENDIX TABLE -.
t There is no special significance in the values after the decimal in these cases, as the lowest pressure observed was in part determined by how near the centers of cer-
tain hurricanes passed to the station, a matter of chance; the probability that the lowest pressure observed is the lowest possible under natural conditions for the region
increases with the number of years of observation. (See also TEXT TABLE 2.)




STONE: METEOROLOGY OF THE VIRGIN ISLANDS 19

The nature of the shorter-period pressure variations in relation to the
weather and the general circulation are discussed below under The Upper
Air and General Circulation, etc.


PRECIPITATION
Rain is the climatic element of most practical concern in the islands be-
cause it is often insufficient to mature sugar cane in one or two seasons; a
drought of six or nine consecutive months occurs every decade or so, caus-
ing much hardship to the townspeople and small native farmers as well as
to sugar and cotton estates and cattle ranches.
Since early in the nineteenth century rainfall in the Virgin Islands has
been measured in a unique unit of depth, called the "line". The reason for
the adoption of this measure is not known. It is an old English measure, in
which 1 inch = 8 lines (= 25.40 millimeters). In Denmark they once used
the Paris measure of 12 Linien = 1 Tomme (Paris inch) = 27.07 milli-
meters-- 1.0658 inches. 1 Paris line = 2.256 mm = .0888 inch = /144
foot, whereas the Danish West Indian (or English) line = 3.175 mm =
1 inch. It is conceivable that as many of the residents were British this
"line" was adopted locally from using English rain-measuring glasses or
sticks graduated in eighths of an inch. Since the American occupation
inches have been used.


Accuracy of the Measurements
The accuracy of rainfall measurements is a difficult problem in gen-
eral, and is especially serious in tropical countries.* We have already re-
ferred to the lack of standards in the instruments and observation pro-
cedures at Virgin Islands stations, and here we must add that where the
rainfalls are frequently light and the monthly and annual totals are small
the errors of measurement are greatest on a percentual basis. The common
practice of measuring the catch only once each 24 hours allows some water
to evaporate from the gage before it is read, particularly in a warm windy
climate. The use of a funnel is common and tends to cut down the evapo-
ration. Where most of the rain falls at night, it is better to read the gage
in the morning, and where it falls more in the day an evening observation
hour is preferable; two readings a day would be still better, and best of all
the use of recording gages or the habit of reading the gage after each
shower. It has been shown that a considerable difference in a given
For a comprehensive discussion see Brooks, C. F., Need for universal standards for measuring
precipitation, snowfall, and snowcover. Trans. Meet. Int. Comm. Snow and Glaciers, Int. Assoc.
Hydrol. Bull. 23: pp. 1-52. Riga. 1938.




SCIENTIFIC SURVEY OF PORTO RICO


month's total may result at the same spot between a gage read each morn-
ing and a gage read each evening. But it is difficult to estimate the magni-
tude of this effect in the Virgin Islands except to say that the results from
gages read only in the morning are probably somewhat lower than they
would be if read only in the evening. The hours of observation at the vari-
ous stations are not stated or known in many cases and at some stations
they were changed from time to time.
Rain gages of different diameter and different height of orifice above the
ground do not give comparable catches, but it is believed nearly all the
gages used in the Virgin Islands since 1870 have been of the standard
8-inch diameter with rim about 3 feet high (cf. appendix A). The wind
eddying around the gage may keep away some of the rain that should go
in the gage. In windy places the catch may average 20 per cent too low
from this cause, but we judge from tests.made elsewhere with shielded
gages that this error in the Virgin Islands probably does not average over
10 per cent (i.e., readings are 10 per cent too low on average from the wind
effect alone). If we may assume that this error applies roughly equally to
all the gages in the Caribbean region, it may be overlooked in practical
comparisons. However, the error due to wind effect increases as the wind
velocity increases and therefore the catch during severe storms, hurri-
canes, is apt to be more than 10 per cent too low. High wind sometimes
blows the gage over resulting in loss of a large catch of rain. Occasionally
during heavy rains the gage may overflow before it is read. Considering all
these sources of error, it is evident that on the average the recorded rain-
falls are systematically lower than the true rainfalls.
In addition there may be mistakes and falsifications on the part of ob-
servers, which are unsystematic in their effect on the results and largely
hidden in the averages. An inspection of the daily entries and the reputa-
tion of the observer are the only bases for accepting observations as genu-
ine, where the stations are not under regular inspection of an efficient na-
tional weather service. We have not found any record of inspections by
the Danish government, and the U. S. Weather Bureau inspections have
been too infrequent to be effective.

General Distribution
From APPENDIX TABLES 2 and 3 we note that the mean annual rainfall
differs considerably at the various stations, ranging between about 35 and
70 inches. The absolute range between driest and rainiest years at these
stations is not much larger, however, the extreme annual totals ranging
from about 25 inches to nearly 95 inches (APPENDIX TABLE 1). If we had





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 21













,AR ESTATES RPOr PTt,
0E !5 -5 50
SCALE OF D.LES

o 40 e o5

FIGURE 2. Rainfall map of St. Croix, 1921-30. (From Shaw, 1932.)





records from eastern St. Croix and from the mountain tops, these ex-
tremes would be greater, probably reaching from 15 to more than 100
inches. A rainfall map of most of St. Croix is shown in FIGURE 2.
The seasonal distribution generally shows two maxima, a smaller one
in May or June and a larger one in October. The winter minimum is
much more pronounced than the summer one. The lowest monthly amounts
on record indicate that severe drought conditions can occur in almost any
month; even October has sometimes had less than 2 or 3 inches at most sta-
tions (see TEXT TABLE 4). Rose points out that the middle and western
sections of St. Croix have somewhat opposite tendencies in departures of
rainfall from normal from 1903 to 1908 the middle was drier than the
west, but from 1909 to 1915 the middle was wetter, and after 1915 the
middle was again the drier. This may possibly be due to a quasi-cyclic
shift in the relative frequency of winds from slightly north and slightly
south of east, which would be accompanied by changes in the average tem-
peratures and humidities of the trade winds as well as contrasted oro-
graphic effects.

Forests and Rainfall
E. Taylor in his "Leaflets from the Danish West Indies" (London,
1888: 42) suggests that St. Croix formerly had a greater rainfall be-





22 SCIENTIFIC SURVEY OF PORTO RICO

cause an early book on the islands by Oldendorp (1777) reported a greater
amount of forest growth than is now found. Although a change of climate
is possible, the present condition is better explained by the known destruc-
tion of the forest by the inhabitants. TEXT TABLE 5 shows no permanent
change in the rainfall of St. Croix since 1852. St. John and Tortola have
the most forests at present because they are too mountainous for economi-

TEXT TABLE 4
FREQUENCY OF MONTHLY RAINFALL TOTALS GREATER THAN
SPECIFIED AMOUNTS, ST. CROIX
Average of 3 stations for 63 years, 1852-1914
(From Ravn)

Month Number of years with rainfall
Over 20 lines Over 40 lines Over 60 lines
(2.50 in.) (5.00 in.) (7.50 in.)
January 25 2
February 13 1
March 13 1
April 33 5 1
May 37 24 11
June 38 19 9
July 44 12 3
August 50 22 8
September 57 32 10
October 60 38 18
November 54 30 13
December 39 11 4

cal sugar-cane culture, though at one time both were under considerable
cultivation. There is no reason to believe that either St. John or Tortola
receive much more rain than St. Thomas or St. Croix merely because they
are now more forested. Indeed, the rainfall observations (cf. APPENDIX
TABLES 2-6) lend no support to that notion.

Orographic Effects
The rainfall increases with elevation on all the islands, as residents and
travelers can readily observe and as one would expect. But rain-gage sta-
tions are lacking at high elevations, except Pearl, Mlafolie, Liliendal, WVint-
berg, and Dorothea. Shaw's rainfall map (FIGURE 2) based on rainfall
records (see APPENDIX TABLE 7) of sugar estates on St. Croix leaves no
doubt that even moderate elevations are better watered. Yet the rate of in-
crease of rainfall with elevation does not here seem to be as large as in the
parts of Porto Rico where the mountains rise steeply to 3000 feet or more
directly in the path of the prevailing winds. Rose suggests that the rain-
fall of the islands is not as great as one would expect from the topography
because the winds blow mostly parallel to the mountain trends. The reason


















TEXT TABLE 5

AVERAGE RAINFALL FOR EACH 10-YEAR PERIOD, 1852-1911 (IN INCHES)*
"St. Croix, Virgin Islands"= (Christiansted's Fort + Kings Hill* + Fredericksted's Fort)/3
(From L. Smith)

Period Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year

1852-61 1.90 1.60 1.68 3.12 5.53 3.76 3.51 4.92 7.26 8.16 4.43 2.68 48.61
W 1862-71 2.11 1.65 2.36 2.06 3.35 3.86 3.10 4.18 5.26 7.80 4.07 3.40 43.00
1872-81t 2.85 1.33 1.57 1.43 4.16 4.48 3.37 4.25 5.28 5.11 6.61 2.33 44.86*
1882-91t 2.78 2.10 1,15 3.27 3.22 3.97 4.06 4.62 4.92 7.50 5.92 3.67 47,50*
1892-1901 2.16 1.45 1.32 2.15 6.33 4.60 5.42 4.58 6.81 5.47 5.46 4.08 49.87
1902-11 2.52 2.31 1.12 2.52 4.26 3.40 2.47 5.40 6.92 4.68 4.96 5.05 47.62

Total 14.33 11.45 10.21 15.57 26.90 24.08 21.95 28.11 36.45 39.38 31.45 21.23 281.35
Average for 60 years
(1852-1911) 2.38 1.91 1.70 2.60 4.47 4.01 3.65 4.70 6.07 6.56 5.23 3.53 46.89

These are from the same observations used in TEXT TABLES 19 to 22, here converted to inches from the "lines" in which rainfall was measured (8 lines = 1 inch).
From "Reports of the Virgin Islands Experiment Station, 1911",
t Kings Hill was omitted from the averages for Oct. 1878 to Oct. 1888, inclusive.




24 SCIENTIFIC SURVEY OF PORTO RICO

for this may also be contained in some observations of the writer: on sev-
eral occasions during his stay at St. Thomas in June, 1939 when the sum-
mit of the island (1800 feet) was visited, he noticed that any large cumulo-
nimbus cloud that had been initiated by forced ascent of the wind over the
island would lean to the leeward so that most of the rain falling from it
would fall on the ocean surface somewhat to the lee of the island. In other
words the orographic influence on the rainfall 'vas not fully enjoyed by the
island itself owidg to its small size and narrow form. This observation is
confirmed (oral communication) by Sergeant Davidovic, the Aerographer
stationed at the U. S. Marine Corps Fleet Air Base on St. Thomas in 1939.
In general the annual rainfall does not seem to increase more than about
10 inches between sea level and 1000 feet elevation, but some of the lower
stations have as much rain as places high up on the leeward slopes or in
high protected valleys (compare Adrian and Cinnamon Bay, or Barracks
and Liliendal, in the same years) (APPENDIX TABLE 2). In generally rainy
years or months the rainfall differences between stations of different ele-
vation are much greater than in generally dry seasons.
At the U. S. Marine Corps station on Lindbergh Bay three rain gages
have been set'a few hundred yards apart in a line from the water to the
foot of the mountain. These gages show a decided increase in rainfall (AP-
PENDIX TABLE 12) as the mountain is approached, although they are all
about at the same elevation. This demonstrates how sensitive the rain-
producing process is to the topography. For this reason, within the hilly
town of Charlotte Amalie, or of Christiansted, the average annual rainfall
probably varies considerably (up to 5 inches?) from block to block; hence
records taken at different spots in such a town cannot justifiably be com-
bined as if from one station. Likewise different parcels of an estate often
have very different rainfall (e.g., Eden, Emmaus, Caroline; Adrian, Su-
sannaberg).
We have not attempted to construct rainfall charts of St. Thomas and
St. John owing to the non-homogeneity of the records. Shaw's map of St.
Croix (FIGURE 2) is based on a homogeneous though short (10 years)
series of 26 records from the flatter parts of the island, which should give
a reliable and consistent pattern.

Year to Year Variation
The variability of the mean annual rainfall is of prime economic conse-
quence because in over half the years the actual rainfall is well below the
normal rainfall,* which is just about sufficient for an annual yield of sugar
It is characteristic of the frequency distribution of either daily, monthly or annual rainfalls,
that the most frequent value (mode) is generally much less than the average, and in some cases the
zero value is most frequent.





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 25

cane, long the chief crop, and the cane yield suffers accordingly (Dr. Shaw
discusses this problem with respect to St. Croix, in paragraphs quoted be-
low). Du Tertre and Oldendorp mention great droughts in 1661 and 1753;
the poor crops of 1841, 1864, 1869, 1872 to 1877, 1891, 1892, 1899, 1904,
1923 to 1924 were due to low rainfall (see TEXT TABLES 20 to 23). Con-
trary to the impression of many residents and travelers, there is no real
evidence that the rainfall is slowly and steadily decreasing from century
to century (see Forests and Rainfall). The question of cyclic variations
has not been scientifically studied here, but results elsewhere generally
show long quasi-periodic fluctuations of considerable amplitude in the
rainfall. These undoubtedly exist here too but the records are not long
enough to reveal any but the shortest'"cycles". The average rainfall is so
near the critical limit for sugar cane that even the small short-period fluc-
tuations are important. It does not contribute much either to fundamental
understanding of the variations nor to practical precautionary measures
for the farmers merely to describe the rainfall curve as quasi-periodic. All
attempts to forecast the fluctuations by means of extrapolating "cycles"
derived from analysis of past records have been failures. Scientific bases
for long-range forecasting are being sought in many directions but the
solutions offered do not yet give results of practical value and general ap-
plicability, however promising the method or enthusiastic the advocates.
The most successful results so far are for certain special conditions and
places, none of which have been in the West Indies.

Diurnal Variation
The diurnal distribution of the rainfall, as at San Juan, shows a much
greater amount of rain by day than by night, judging from Mr. A. Wal-
1Ie's observations at Charlotte Amalie, published in the "Set. Thomae
Tidende", 1888. He gives the following figures.

TEXT TABLE 6
NIGHT AND DAY RAINFALL, CHARLOTTE AMALIE, 1888*

Month (1888) Total By day By night
July 38.4 26.8 11.6
August 77.2 55.9 21.3
September 69.0 44.6 24.4
In lines; 8 lines = 1 inch.

The frequency of rain is no doubt also greater by day but the contrast
is probably not so pronounced because the intensity of the day showers is
heavier.




26 SCIENTIFIC SURVEY OF PORTO RICO

At sea the rainfall frequency is a maximum at 6 A.M. with a secondary
maximum at about 10 P.-M. The amplitude of this daily variation is pre-
sumably smaller than the one observed over the islands, where the maxi-
mum comes in the afternoon. It is very likely that the sea maximum at
6 A.M. affects the islands, or at least their shoreward margins, causing a
secondary maximum at that hour. No hourly observations are available
from the islands but the sunrise shower seems to be recognized by the
residents as a more or less regular phenomenon. The daily double period
in the rainfall is of course reflected in the cloudiness (TEXT TABLE 17) and
in the frequency of thunderstorms.


Intensity and Frequency
The rainfall in this low latitude and oceanic situation is entirely of the
shower type, and therefore it is of great practical importance to know how
frequently showers occur, how long they last, how much rain falls per
shower, and what are the average and maximum rates of fall over short
periods of time. Unfortunately systematic observations using recording
rain gages were begun in the islands only very recently, so we are forced
to infer much from the usual rainfall observations which give only
monthly totals and numbers of rainy days. The average rainfall per rain
day (APPENDIX TABLES 10 and 12; FIGURE 10) indicates some important
characteristics.
The "showers" of the winter and spring seasons are characteristically
brief and light, often mere sprinkles, from cumulus clouds of small or
moderate size and spaced by large intervals of blue sky (cf. TEXT TABLE
9). Sometimes northerr" effects cause a low overcast cloud deck with driz-
zling rain punctuated by occasional heavier showers, which condition may
persist a day or two. However, very heavy rains up to 2 or 3 inches in a
day have fallen even in the driest months. In the "rainy season", from
May to November, heavier and more enduring showers, with squalls or
thunder and lightning at times, are to be expected much more often; at
least one shower of some sort then falls almost every day. Heavy rains
lasting as much as 6 or 8 hours, even with brief intermissions, are normally
very rare, but passage of a hurricane within 50 or 100 miles can cause enor-
mous rainfall totals (over 10 inches) in a day or two from virtually con-
tinuous downpours. The high wind during hurricane weather adds greatly
to the destructive effect of the rain.
Some significant deductions can be made from the results of the re-
cording rain gages, in spite of the short period they have been in use.
At the Marine Barracks of Bourne Field on St. Thomas a recording rain




STONE: METEOROLOGY OF THE VIRGIN ISLANDS 27

gage has been operated since 1935. An analysis of the results (API'PN, X
TABLE 12 and FIGURE 10) indicates that the average rainfall per rain day
and also per rain hour for each month is proportional to the greatest rain-
fall (luring any 24 hours of the corresponding months. This is a very in-
teresting relation because in the absence of recording rain gages at other
places we can assume that the "greatest rainfall in 24 hours", which is
tabulated by the U. S. Weather Bureau for all its stations, gives a rough
basis for estimating the average intensity of rainfall per day and per hour.
Since February 1940, the Soil Conservation Service has been tabulating
rainfall rates monthly from recording gages at Anna's Hope and Jolly Hill
estates on St. Croix. An abstract of the results appears in TEXT TABLES 7
and 8. Although the period of observation is too short to give any definite
averages or extremes likely to occur, the figures are already significant. A
study of the tables reveals a closer correlation by months between the total
rainfall and the maximum intensities than between the total rainfall and
the average intensities. This is not surprising because one or two intense
showers probably have more effect on the monthly totals than the more
numerous lighter showers. There is nevertheless some tendency for the
average intensity to be greater in the rainier months than in the drier
months. It will be noted, however, that the average intensity in the spring
months appears to be as high as or higher than in the autumn months,
whereas the total rainfall is usually much greater in the autumn than in
the spring. This is a curious fact which we have already suspected from
the greater frequency of hail in the late spring and early summer than in
the late summer and autumn. Over a period of many years the average in-
tensity of rainfall will actually be greatest in the autumn or late summer
because of hurricanes. The important conclusion is that, if hurricanes are
excluded, winter and spring showers probably have as great average in-
tensity as the autumn rains, but the maximum rates of rainfall in short pe-
riods, as shown in TEXT TABLES 7 and 8, are generally two or three times
greater in the "rainy season" than in the winter and spring. It is impossible
to infer to what extent this conclusion is justified for all parts of the
islands, as the topography may greatly affect the rainfall intensities as
well as the totals, but the Bourne Field results (APPENDIX TABLE 12) seem
to show similar features to those of Anna's Hope and Jolly Hill estates.
Any practical interpretation of the average rainfalls reported in the
Virgin Islands, especially on St. Croix, should take into account the fact
that a large proportion of the rain falls in light showers and brief sprinkles
(see TEXT TABLES 9 and 10). Many of these light rains are measured in the
rain gages and they augment the total rainfall out of proportion to their
significance for crop growth and for vegetation because they barely wet





28 SCIENTIFIC SURVEY OF PORTO RICO


TEXT TABLE 7

RAINFALL INTENSITIES \MEASURED AT
STATION SCS No. 18 F. S. A., JOLLY HILL ESTATE, ST. CROIX, V. I.

(From U. S. Soil Conservation Service)


Total Total Average Maximum Intensity for Different Intervals
Rainfall, Duration, Intensity,
Month inches hours* in./hr. 5-min. 10-min. 20-min. 60- min. 120-min.


1940
February
March
April
May
June


0.90 15.02 0.06
0.52 1.92 0.27
1.97 6.42 0.31
7.10 30.00 0.24
3.05 8.10 0.38


July 2.14 5.07 0.42
August 3.05 12.37 0.25
September 4.19 12.65 0.33
October 7.47 22.45 0.33
November 7.15 25.37 0.28
December 3.47 20.25 0.17


1941
January
February
March
April


5.02 0.39
0.80 0.26
1.13 1.17
9.45 0.24


1.00 0.75 0.35 0.13
2.00 1.50 0.80 0.50 0.28
3.50 2.00 1.40 1.10 0.70
2.00 1.50 0.90 0.55 0.30
5.00 3.50 2.80 1.30 0.65
3.00 1.75 0.95 0.30 0.15
7.00 5.00 3.40 1.40 0.75
4.50 2.75 2.80 1.70 1.05
5.00 3.50 2.40 1.45 0.90
3.50 2.25 1.60 0.85 0.45

2.00 1.75 0.90 0.40 0.20
1.50 1.00 0.50 0.18 -
3.75 2.50 1.40 0.56 0.30


* Intensities of less than 0.10 in./hr. are not included.




TEXT TABLE 8


RAINFALL INTENSITIES MEASURED AT
STATION SCS No. 15 F. S. A., ANNA'S HOPE ESTATE, ST. CROIX, V. I.

(From U. S. Soil Conservation Service)


Total Total Average Maximum Intensity for Different Intervals
Rainfall, Duration, Intensity,
inches hours* in./hr. 5-niin. 10-min. 20-min. 60-min. 120-min.


1940
January 0.35 2.77 0.13
February 2.09 17.18 0.12
March 0.99 4.92 0.20
April 1.55 4.20 0.37
May 2.88 15.05 0.19
June 1.56 4.60 0.34
July 1.17 4.23 0.28
August 1.75 7.48 0.23
September 5.24 6.67 0.82
October 8.43 22.05 0.38
November 6.05 14.67 0.41
December 2.36 14.10 0.17
1941
January 3.67 12.25 0.30
February 0.19 2.50 0.08
March 1.05 2.83 0.37
April 2.48 7.25 0.34


1.80 1.25 0.75 0.30 0.18
1.00 0.75 0.35 0.13
3.50 1.75 1.20 0.80 0.43
2.00 1.50 0.80 0.40 0.25
3.00 2.25 1.20 0.40 0.20
1.00 0.75 0.35 0.13
3.00 2.00 0.80 0.30 0.15
7.00 5.00 4.20 2.30 1.18
4.00 3.00 2.00 0.80 0.50
7.00 5.50 3.30 1.10 0.60
1.50 1.00 0.45 0.18 -

4.00 2.50 1.60 0.60 0.38
1.50 1.00 0.50 0.20 0.15
4.00 3.00 2.60 0.92 0.48


* Intensities of less than 0.10 in./hr. are not included.


Month





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 29


the vegetation and the top of the soil and do not sink into it, and so are
quickly evaporated by the sun and wind.




TEXT TABLE 9

PERCENTAGES OF DAYS WITH SPECIFIED AMOUNTS OF RAINFALL.
CHRISTIANSTED, ST. CROIX, 1852-1907
(From Willaume-Jantzen and Ravn)


Month

January
February
March
April
May
June
July
August
September
October
November
December

Year


0-5 mm
(0-0.20")

67
64
66
55
56
46
55
54
45
44
48
52

54


> 20 mm
(0.79"
or more)

5
3
4
16
10
14
10
11
15
18
12
8

11


> 50 mm
(1.97"
or more)

0
0
0
2
4
3
2
4
4
7
3
2

3


TEXT TABLE 10

AVERAGE AND EXTREME NUMBERS OF DAYS WITH RAIN, CHRISTIANSTED,
ST. CROIX, 1852-1907

(From Willaume-Jantzen)


Highest Lowest
in any one year in any one year


These figures are not sums of the columns above, but are the extreme totals on record for any one
year in the period covered by the table.


Month

January
February
March
April
May
June
July
August
September
October
November
December

Year




30 SCIENTIFIC SURVEY OF PORTO RICO

Evaporation
The actual water loss from the ground by evaporation and by transpira-
tion of plants is probably high, judging from the general weather condi-
tions and from the measures of evaporating power of the air made at the
Experiment Station (see APPENDIX TABLE 8). Consequently, the roughly
45 inches of measured average annual rainfall in the Virgin Islands is by
no means the equivalent for plant growth of 45 inches of measured pre-
cipitation in rainier parts of the West Indies or in the southern United
States.

Thunderstorms, Squalls, and Hail
Thunderstorms occur, as in Porto Rico, chiefly from July to October,
according to the records at Christiansted and Bourne Field (TEXT TABLE
1 and APPENDIX TABLE 12). Schomburgk in 1837 reported that 5 to 10 per
cent of the days in a year had thunderstorms, mostly in September aid
October, which roughly agrees with the Christiansted data, although at
Bourne Field more of the storms occur in July and August. Most storms
probably occur in the afternoon, as at San Juan. They are apt to be squally
and inflict wind damage at times, but lightning damage is usually slight.
Squalls are sometimes associated with heavy showers and probably with
most thunderstorms. The familiar downrush of cold air under a thunder-
storm or tall cumulonimbus cloud can be so violent as to capsize small
boats and damage dwellings, trees, and crops. When the observer is located
on the sunny side of the cloud, it may appear white until after the squalls
reach him, giving rise to the term "white squall" of the West Indian na-
tives; but when the observer is under or on the shaded side of the cloud,
it appears very dark and ominous, so the accompanying gusts are called
"black squalls". White squalls are also reported without heavy clouds
nearby, but these are merely gusts when the trades are blowing strongly.
The West Indian sailor well knows that the squalls are apt to be especially
violent and dangerous to boats along a coast which rises to high mountains
immediately back of the shore.
Hail is rarely reported and most residents spend a lifetime in the islands
without seeing any. There are enough authenticated reports to leave no
doubt that it falls at least every few years, even several times in some years
in which conditions are favorable for it. Much hail, with cold and rainy
weather, occurred in Virgin Gorda in January 1833, according to Schom-
burgk, who also wrote of hail on the north side of Tortola in November
1829. Knox mentions that hail as big as hen eggs fell in St. Croix on
April 13, 1844; and that a Mr. Nissen told him of a hailstorm at St.




STONE: METEOROLOGY OF THE VIRGIN ISLANDS 31

Thomas on May 13, 1829. A reliable eyewitness reported that hail fell in
St. Thomas in 1938. Although in Porto Rico most hail comes in spring and
early summer, the cases cited for the Virgin Islands are chiefly in the win-
ter and spring; perhaps the winter hailstorms are more phenomenal and
thus more likely to be remembered or noticed.

Chemistry of Rainwater
Chemical analyses of rainfalls were regularly made at the Experiment
Station from 1911 to 1915 (see TEXT TABLE 11). Rainwater (fresh) con-
tained an average of 9.6 parts per million of chlorides, 0.262 parts of
nitrogen in the form of ammonia, and 0.324 parts of nitrogen as nitrates.
These figures varied greatly from month to month and storm to storm.
The amounts do not seem to have a definite seasonal variation, nor do they
appear to depend on the total monthly rainfall, though it is quite possible
that they are related to the intensity and amount of the individual shower.
These chemical constituents of the rain have an important effect on the
soil and the nourishment of crops, a subject much studied by agronomists.

Water Supply and Irrigation
Owing to the small and erratic rainfall, the dry porous soils, high evapo-
ration, and the few permanent streams, it has been a serious problem to
obtain domestic water supply. Rain water is diligently caught from roofs
and stored in cisterns, and several acres of steep slopes are paved with con-
crete to catch rain for use by the town people. Drought periods enforce
strict economy in use of water and sometimes necessitate importing water.
Shallow dug wells are used for cattle and gardens, and salt water is
pumped for flushing the sewers. The cisterns for drinking water are
stocked with "mosquito fish" and screened, for in the past malaria was
spread chiefly by mosquitoes that bred in uncovered cisterns. Irrigation
of La Grange plantation from a dammed mountain brook in northwestern
St. Croix was started in 1910 and perhaps similar projects are feasible,
but not on a scale sufficient for all fields during a drought. Deep wells have
not yet been tried. Stored surface water might deposit alkali or salt in soils
on which it was used for irrigation.

TEMPERATURE
Geographical Controls
Temperatures in the Virgin Islands are more uniform than at most
Porto Rican stations owing to the more insular exposure and relatively
small land area available for local radiational and insolational influences.





32 SCIENTIFIC SURVEY OF PORTO RICO




TEXT TABLE 11

CHEMISTRY OF RAINFALL AT ANNA'S HOPE, ST. CROIX (EXPERIMENT STATION)

(From Reports V. I. Exp. Sta., 1911-15)


Parts per million of


Nitrogen as Nitrogen as Nitrogen
ammonia nitrates total


Date

1911
Sept.
Oct.
Nov.
Dec.

1912
Jan.
Feb.
March
April
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.

1913
Jan.
Feb.
March
April
May
June
July
Aug.
Sept
Oct.
Nov.
Dec.

1914
Jan.
Feb.
March
April
May
June
July
Aug.
Sept,
Oct.
Nov.
Dec.

1915
Jan.
Feb.
March
April
May
June

Average


Rainfall
in lines
(1 line = Y/ in.)


29.8 }
50.3
17.0
43.6


10.1
19.5
14.9
12.2
10.9
33.5
22.581
10.24
6.726
75.28
70.32
10.96


28.241
12.645
21.361
32.005
66.72
3.68
19.20
14.88
29.60
42.40
18.16
19.60


21.76
14.40o
9.608
7.761
86.96J
28.00
11.040
19.045
10.641
20.401
38.321
23.283


11.28
6.88j
20.001
102.00
80.40
95.20


0.180 0.710
.......... contaminated ..........
0.400 0.810
.......... contaminated .........
0.440 0.320
0.160 0.230


0.100 0.280
0.400 0.260

0.320 0.110
.......... contaminated .........
0.240 0.450
0.500 0.340

0.200 0.120


- 9.6 0.262 0.324


0.290
0.260
0.290



0.370
0.390
0.440
0.070
0.420

0.100



0.180
0.370

0.100


Chlorine


0.234
0.074
0.124




0.310
0.254
0.500
0.220

0.380



0.274
0.423
0.266


0.524
0.334
0.414




0.700
0.694

0.570
0.640
0.480



0.454

0.793
0.366
0.890
1.210

0.760
0.390


0.380
0.660
0.430

0.690
0.840
0.320



0.210
0.460
0.240
0.240
1.180

0.586





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 33

The climate is fairly sunny, and the insolation on some days, especially in
spring, is very intense.
Owing to the small number of temperature station records and their
lack of homogeneity, it is not feasible to construct reasonably accurate iso-
thermal charts of the islands. However, a true isothermal map would show
very much the same pattern as the topographic contour map, because the
mean temperature varies chiefly with elevation, though the vertical gra-
dient varies somewhat with exposure and distance from the coast (cf.
Vertical Temperature Gradient, infra). The geographical distribution of
the temperature shows much less range and complexity than the rainfall
distribution and is of correspondingly less practical concern.
The nearness of the sea and the prevailing trade winds prevent excessive
maximum temperatures from local insolational heating in the interior. The
mean temperatures (computed from observations recorded in the standard
louvred thermometer shelter) do not differ much from place to place, and
are controlled chiefly by the temperature of the ocean surface to the wind-
ward, and by elevation (APPENDIX TABLES 9-10 and TEXT TABLE 15). In
sheltered interior valleys such as Anna's Hope, St. Croix, the night mini-
mum temperatures are somewhat lower and the wind dies down more at
night than at coastal points or on exposed slopes. Dew is frequently re-
ported on clear nights especially at such interior places, and is not unknown
at any point.
It is the impression of many residents and students of the Virgin Islands
that St. Croix is somewhat warmer than the other islands. There is no
doubt that on any of these islands the windward sides must be, and do
"feel", somewhat cooler than the leeward sides, if only because of a con-
trast in cloudiness and wind velocity. Consequently, when comparing
Christiansted or Fredericksted with Charlotte Amalie, the latter is usually
rated somewhat cooler, although the mean temperatures are similar. It has
even been thought that St. Croix as a whole is noticeably warmer because
of its more southern location. There is as yet no observational evidence
from sea-water nor air-temperature data to justify such a conclusion, and
if it be qualitatively true, the magnitude of the difference must be small. As
St. Croix is only 40 miles south of St. Thomas, equal or greater differences
are to be expected from one place to another on any of the islands, except
possibly the smallest "rocks". It is quite misleading to attempt to differ-
entiate climates within the islands from the available weather records.*
Owing to the fluctuations of the mean temperature above and below the normal from year to
year, the averages of the shorter-period weather records are probably not representative and should
not be compared with the longer records without some allowance for the trends. About 10 years are
required to give an acceptable approximation to the temperature normal under tropical conditions,
but 30 years are needed to smooth out the small-amplitude so-called sunspot-cycles, while a large-
amplitude cycle of about 100 years must be reckoned with in most climates. (Rainfall fluctuates
much more than temperature, so that 30 to 100 years are needed to give a reliable normal.)




34 SCIENTIFIC SURVEY OF PORTO RICO

The limited area of forest cover on St. Croix, compared to well-wooded
St. John and Tortola and partly-wooded St. Thomas, is certainly a factor
that may tend to elevate the temperature. In the mean temperatures for
the whole 24 hours of the day, the effect of the greater insolational heating
of a barren land is offset to a large extent by a greater radiational cooling
at night. Thus we may expect, other things being equal, St. Croix to have
a greater daily range of temperature than the other islands. The relative
flatness and somewhat greater extent of St. Croix also favor a greater
daily range. Comparing the temperature records (APPENDIX TABLES 9 and
10), note that Anna's Hope in central St. Croix has an annual average
daily range of 14 F., whereas the coastal stations have about 9 or 100
daily range. The mean minimum temperature at Anna's Hope is slightly
lower than at any other station and the mean maximum slightly higher.


Extremes: Cool Spells and Warm Spells
The coldest winter nights at Anna's Hope generally follow immediately
after a series of abnormally warm nights (minima above 750 F.). A slight
shift of wind takes place between these episodes from south of east to
north of east. The warm nights are due to the deep current of moist air
imported with the southeast component of the wind, which prevents much
cooling of the ground by radiation at night; but the cold nights are due to
the drier upper air and lack of clouds that follow in the wake of the
northerly component in the trades. The first night with more northerly air
may remain quite warm due to low clouds resulting from cooler air pass-
ing over the warm sea, but the second and third nights will usually clear
off and become cool unless the normal air circulation has completely re-
established itself by this time. It is not unusual for three or four nights in
succession to have minima of 600 F. or less in the winter. Schomburgk
mentions a minimum of 360 F. occurring once at Tortola in the early part
of the last century, which caused the natives much suffering, but such a low
temperature seems impossible and must be a misprint for 560 F. It will be
recalled, however, that temperatures were abnormally low in the United
States in the early decades of the 19th century, when there must have
been many severe northerss" in the Caribbean.*
The weather of January 16-20, 1914, is an excellent, if somewhat ex-
*The "Set. Thomae Tidende" for March 15, 1837, reported, "The weather has been cool for
some time past, and of late we have had frequent stiff breezes from the north [sic!], both probably
arising from the prevalence of cold weather in America". On March 22 of the same year the
"Tidende" related, ". . the Packet Express . arrived here late on Monday, in 15 days from
Barbados, being, it is said, the longest passage ever known, of either mail boat or packet from
windward . it is accounted for by the strong northerly wind that has prevailed for some time
past".





STONE: METEOROLOGY OF THE VIRGIN ISLANDS


treme, example of a cold winter night between rather warm nights, as is
shown by the data of TEXT TABLE 12.

TEXT TABLE 12
TEMPERATURE AND RELATIVE HUMIDITY, JANUARY 15-21, 1914
AT ANNA'S HOPE, ST. CROIX
(Data from V. I. Agric. Exp. Sta. Report, 1914)

Date Mlin., A.M. 8:30 A.M. 12 noon Max., P.M. 4 P.M. 9 P.M.
Rel. Rel. Rel.
1914 Temp. Temp. Hum. Temp. Hum. Temp. Temp. Hum. Temp.
January F. F. % F. % F. F. % F.
15 70 79 80 81 81 82 79.5 84 74
16 66 74 88 78 76 80 78 68 71
17 68.5 75 72 78.5 72 79.5 78 66
18 52 64 79.8 80 79.5 59.5
19 [55?] 77 78 81 88 86 75 89 75.5
20 74 76 81 82 86 85 82 76
21 68 78 79 82 83.5 80 72 70

This is a striking example. Note how the temperature failed to fall be-
low 68.5' on the early morning of the 17th in spite of the rather low hu-
midity which had already set in on the afternoon of the 16th. The warm
night was due to clouds which also kept the temperatures from getting ab-
normally high during the day. The night of the 17th was clear and cold
(52 min.) and likewise the next night (18th), with continuing low hu-
midity. The 19th and 20th were clear and the sun heated the air up to 86
and 85. The humidity rose again on the 19th and it was cloudy that night
so that the minimum on the morning of the 20th was high, 74. No rain
fell from the 16th to the 20th.
Long cloudy, rainy spells sometimes reduce the daily range of tempera-
ture to less than 50, a disagreeable type of weather in the tropics, but it
does not happen as a rule more than five to ten times a year in the Virgin
Islands.
What might be called hot spells occur practically every year, particularly
between May and November. These are times when the daily maximum
temperature exceeds 88 or 90 F. for several days in succession, especially
when the humidity is unusually high and the wind light at the same time.
The daily maximum at low-level stations holds rather constantly from 83
to 880 F. in the warmer half of the year, wherefore an occasional day with
90 or 950 F. is very noticeable to the residents. Spells with daily maxima
above 90 F. are disagreeable not so much because of the maxima as be-
cause of the minima, which are often about 78 to 800 F., so that the nights
do not allow one to recuperate from the heat of the days.





36 SCIENTIFIC SURVEY OF PORTO RICO

Character and Effects of Changes
Changes from season to season, from day to day* and night to day, or
because of storms, wind-shifts, or showers are small as compared to those
in high latitudes and in more continental parts of the subtropics, but they
are important to the health and comfort of more or less permanent resi-
dents in the islands. This was noticed long ago, and is mentioned by many
writers. Knox (1852) noted a sudden drop in temperature of 5.4 F. dur-
ing the passage of a shower, but this is unusual. Nevertheless, he says it
had as serious an effect on the health of the inhabitants as a drop of 260 F.
would in New York, because "influenza, ague, or bowel complaints, etc.,
succeed such falls of temperature". In July and August 1851, 4000 of the
population were affected by influenza. Knox wondered if the epidemic
were not caused by the few cool hours of rain which followed a period of
more than usual heat and drought a theory which may be discounted
in this case as epidemic influenza, once begun, rapidly spreads to all cor-
ners of the earth as fast as the germs can be transported. Nor can we agree
with Knox's conclusion that, "It is owing to the very minute daily varia-
tions that this climate is healthy, and so happily adapted to the individuals
suffering under pulmonary attacks."
The chief drawback to this climate is the lack of stimulating variations,
which lowers the resistance to unusual changes when they do come and
permits a marked slackening of tone in those who fail to return to the
higher latitudes at intervals.t Indeed, many residents and physicians (and
Knox himself) have admitted that a change to a temperate climate every
few years is almost as necessary in the present relatively hygienic era as
in the more disease-ridden Danish times.
Schomburgk observed at Tortola a very unusual drop of temperature
on April 29, 1832, from 79.50 F. at 9:15 A.M. to 730 F. at 9:25 A.M. to
710 F. at 12:15 P.M. ; there was a quick drop of 6.5 F. in 10 minutes, and
of 8.50 F. in 2 hours.


Daily Cycle
Of course the regular diurnal temperature changes are a source of
physiological stimulation. Everywhere they average less than 15 F. and
at many localities are less than 100 F. (see APPENDIX TABLES 9, 10, and 12),
but for the individual days the range varies considerably depending on the
The mean interdiurnal (day to day) temperature variability at San Juan is a little more than
1" F. in all months. The figures for the Virgin Islands stations would be about the same, and cer-
tainly less than 2* F. even at interior locations.
t Stone, R. G. "Some results of modern physiological research in relation to acclimatization in
the tropics". Appendix 1 in Price, A. G. "White Settlers in the Tropics". New York, 1939.





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 37

amount of clouds, wind velocity, and rains. Normally the hottest hour
comes between noon and 2 P.M., perhaps averaging a little later in winter
than summer; sunrise is the coolest (see TEXT TABLES 13 and 14). In clear
weather the diurnal cycle is surprisingly regular, but irregularities in the
thermograph traces are common, due to the passage of clouds and rain,
and to changes in wind velocity or direction. There seem to be various
characteristic types of these daily curves. Knox illustrates three from the
month of September 1845, and Stenzel's 1886 figures are interesting. The
many curves for Anna's Hope in the Experiment Station Reports for 1911
through 1915 show all the likely variations.
Especially noteworthy is the characteristic alternation of spells of con-
trasted types of daily temperature cycles, for example:


(A) small daily range
1. high maximum temperature and
high minimum temperature (due
to clouds at night?)
2. low maximum and low minimum
(due to heavy rain or clouds by
day?)
3. low maximum and high mini-
mum (due to steady rain, and
clouds day and night)


(B) large daily range
1. high maximum temperature and
low minimum temperature
(clear day and night?)
2. normal maximum and very low
minimum (dry air, some clouds
by day, none at night)
3. very high maximum and normal
minimum (dry air, light wind
and clear by day, partly cloudy
or humid and windy by night)


Other combinations are also observed. A curious secondary rise in tem-
perature often appears around 2 A.M. and abates by sunrise, perhaps due to
a night-time increase in the wind velocity. The uniformity of the daily
range from day to day is closely related to the regularity of cloudiness.



TEXT TABLE 13
STE.-CLAIRE DEVILLE'S TEMPERATURE OBSERVATIONS AT ST. THOMAS
DURING 1840 (BROKEN SERIES)*


Temperature Hours of
Mean difference extremes
* temperature 6 A.M.-I P.M. (approx.)


25.5 C,
77.9* F.
27.60 C.
81.70 F.
26.80 C.
80.2 F.


4.20 C.
39.6 F.
4.1 *C.
39.40 F.
3.2 C.
37.8 F.


Hours at which the
mean temperature
is reached


6 A.M., 1 P.M. 6:36 P.M
6 A.M., I P.M. 6:36 P.M.
6 A.M., 1 P.M. .6:36 .M.


Hornbeck's averages of 5 years' observations at Charlotte Amalie, 1829, 1830, 1834, 1835, 1836, are:
77.9 F. at 7 A.M., 81.7 F. at 4 P.M. and 78.8 F. at 8 P.m. These seem to give too small a range, prob-
ably owing to defective exposure of the thermometer.


Months

March-April
July-August
October-November





38 SCIENTIFIC SURVEY OF PORTO RICO

TEXT TABLE 14
MEAN TEMPERATURES AT DIFFERENT HOURS AT CHRISTIANSTED,
ST. CROIX, 1913-15
(From "Meteorologisk Aarbog")

Year 8 A.M. 2 P.M. 9 P.M.

1913 26.0' C. 27.6' C. 25.3' C.
78.8' F. 81.70 F. 77.5 F.
1914 26.2' C. 28.4* C. 25.7 C.
79.2 F. 83.1* F. 78.3* F.
1915 26.70 C. 28.30 C. 26.1 C.
80.10 F. 82.90 F. 79.0* F.


Vertical Temperature Gradient
The expected decrease of temperature with elevation is definitely ob-
servable in the range of elevation from sea level to over 1000 feet, but this
amounts to only a few degrees on the average (compare Canaan, Wint-
berg and Charlotte Amalie, St. Thomas; and St. Bernard's and Roadtown,
Tortola). Ste.-Claire Deville in 1840 found that the average gradient was
1 C. per 100 meters up to 500 meters. Knox made (1845?) the following
comparisons between Charlotte Amalie and Louisenhoj (778 ft.) :

TEXT TABLE 15
TEMPERATURE DIFFERENCE BETWEEN CHARLOTTE AMALIE AND
LOUISENHOJ, ST. THOMAS

Charlotte Amalie Louisenhoj Difference
Hour (20 -100 feet?) (778 feet) (ca 700 feet)

6 A.M. 76.1* F. 72.1* F. 4.0 F.
2 P.M. 83.8 79.1 4.7
8 P.M. 78.8 73.7 5.1

These differences are rather large and lead us to suspect they may rep-
resent unusual days or else the thermometers or their exposures were not
comparable or proper. On the other hand, since Louisenhoj is on a sum-
mit well ventilated by the trade winds, and Charlotte Amalie is sheltered
therefrom and overheated by virtue of its stone surfaces, it is conceivable
that these "superadiabatic" gradients can exist much of the time. Airplane
soundings made from Bourne Field (APPENDIX TABLE 17) also reveal
slightly superadiabatic lapse rates for the lowest 500 meters in the sum-
mer months (8 A.M.), but those data are open to various interpretations
and criticisms.*

See McDonald, Bull. Am. Met. Soc. 23: 75-76. Feb. 1942; Conrad, V. Meteorological results
of the "Meteor" expedition 1925-27. Bull. Am. Met. Soc. 23 (4): 143-147. 1942.





STONE: METEOROLOGY 01 THE VIRGIN ISLANDS 39


HUMIDITY, SUNSHINE, AND CLOUDS

Absolute Humidity

The absolute humidity may be judged here from the vapor-pressure ob-
servations because the barometric pressure varies so little. TEXT TABLE 17
gives the values for Christiansted. The specific humidity (grams of water


TEXT TABLE 16
MONTHLY MEAN HUMIDITY

Period Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year

Relative Humidity, per cent of saturation
Frederikssted, 7 A.M. mean
1879 77 79 81 80 85 84 78 83 83 85 83 (77) 81
Christiansted, mean (8 A.M. + 2 P.M. + 9 P.M.)/3
17 years
(1900-16) 74 73 72 72 74 76 74 75 77 78 78 77 75
Bourne Field, mean (8 A.M. + 12 M. + 4 P.M.)/3
4 years
(1935-39) 73 70 68 72 73 72 72 74 71 74 74 74 72

Specific Humidity, grams per kilogram
Bourne Field, 8 A.M.
1937 14.9 15.4 14.8 15.7 17.5 17.6 17.6 18.7 19.2 16.6 18.4 15.4 16.8



TEXT TABLE 17
VAPOR PRESSURE, RELATIVE HUMIDITY, AND CLOUDINESS AT CHRISTIANSTED
FOR DIFFERENT HOURS OF THE DAY, 1913-15
(From "Meteorologisk Aarbog")

Vapor Pressure Relative Humidity Cloudiness (in tenths of
Year (mm.) (per cent) sky covered)

8 A.M. 2 P.M. 9 P.M. 8 A.M. 2 P.M.* 9 P.M. 8 A.M. 2 P.M. 9 P.M.

1913 18.4 18.7 18.7 74 68 78 4.1 4.5 3.0
1914 19.2 19.2 19.5 75 67 80 4.2 3.8 3.7
1915 20.4 20.5 20.4 78 71 81 4.9 4.7 4.0

Mean 2 p.M. relative humidity for 4 years was 70 per cent, ranging from 65 per cent in March to
75 per cent in November; see TEXT TABLE 1 for details.

per kilogram of moist air) is also given for Bourne Field in TEXT TABLE
16. After 1916 no vapor pressures are available from the Virgin Islands,
but San Juan, Porto Rico, values may be taken as a rough indication of
mean conditions at sea level in the islands.




40 SCIENTIFIC SURVEY OF PORTO RICO

Relative Humidity
At Christiansted, the mean annual relative humidity (TEXT TABLES 16
and 17) for the hours 8 A.M., 2 P.M., and 9 P.M is 3 per cent lower than
that for San Juan, Porto Rico; the 2 P.M. mean (TEXT TABLE 1) is 6 per
cent lower than the noon mean for San Juan. It seems likely that these
figures indicate a real, though small, difference in the respective climates,
which might be expected from the lower rainfall and more sheltered situa-
tion of Christiansted. The four-year record at Bourne Field, St. Thomas
(sea level, southwest coast), shows a mean annual (8 A.M. + 12 M. +
4 P.M.)/3 relative humidity (TEXT TABLE 16) of 72 per cent; the (9 A.M.
+ 9 P.M.)/2 mean for these years at San Juan was 80 per cent, and the
8 A.M. mean at St. Thomas was about 5 per cent below the 9 A.M. mean of
San Juan for the same period; thus St. Thomas is appreciably drier than
San Juan.
Even where the rainfall supports only cacti, the humidity is probably
still rather high (but 5 to 15 per cent below that at sea) owing to the
proximity of the sea and the constant draft of moist ocean air circulated
over the islands by the trades. This high humidity and the salt content of
the sea air naturally cause corrosion and deterioration of metal, furniture,
and buildings, though by no means so severe as in the rainier tropical and
equatorial climates. The average diurnal range of the relative humidity is
certainly less than 15 per cent, since at both Bourne Field and Christian-
sted the difference between 8 A.M. and 12 noon or 2 P.M. readings is well
under 10 per cent. The highest humidity may be expected at sunrise, when
the temperature is lowest; and the lowest between noon and 2 P.M., when
the temperature is highest. October and November in the rainiest season,
and March in the driest season are the periods of highest and lowest aver-
age humidity respectively, but the difference is only 6 per cent or less.
(NOTE: The hygrodeikk" readings published in the St. Croix Experiment
Station Reports, 1911-15, are from an unventilated instrument, and
not comparable to, nor as accurate as, the standard ventilated psychrometer
observations.) Typical day to day variations are illustrated in TEXT TA-
BLE 12.

Sunshine and Radiation
No measures of solar radiation are available but the intensities probably
do not differ greatly from those at San Juan where several years' records
are available. Slightly more insolation than at San Juan may be expected
due to lesser cloudiness and humidity. Whether there is an abnormally
early diurnal maximum of total radiation and sunshine (between 11 A.M.
and noon) in the rainy season, as at San Juan, cannot be surmised.




STONE: METEOROLOGY OF THE VIRGIN ISLANDS 41

Cloudiness
Records of cloudiness from Christiansted and Bourne Field (see TEXT
TABLE 1 and APPENDIX TABLE 15) are probably reliable. The Charlotte
Amalie record is short and hence erratic (TEXT TABLE 2). The tables (AP-
PENDIX TABLE 11) of clear and cloudy days for the other stations should
not be relied upon too closely as the different observers were allowed to
judge for themselves what constitutes a "cloudy day", etc. (In Porto Rico,
the results of such observations were found to be very misleading.) In
general, the cloudiness (and, complementarywise, sunshine) varies con-
siderably with location where there are mountains sufficient to interfere
with the trades. The windward side of the upper slopes frequently is
shaded by, or shrouded in, clouds, especially in the rainy season; however,
one can observe in the Virgin Islands how the large cumulus clouds which
originate where the air is pushed over the mountains lean far out to the
leeward and shade the lee side as much as or more than the windward. It
is recorded in a book cited by Rose that Crown Mount, 1750 feet high, is
often in the clouds during the rainy season. Judging from the Christian-
sted (TEXT TABLE 17) and Bourne Field records (APPENDIX TABLE 15),
the amount of clouds, or rather the percentage of the sky covered by them,
averages only slightly greater during midday and afternoon than in morn-
ing and early evening. However, these are coastal stations whose horizons
encompass a large part of the sky over the water where the afternoon
clouds from the land may not reach. The real diurnal range of cloudiness
over the islands proper is certainly greater than the figures available sug-
gest. The diurnal range of variation in amount of cloud cover is as great
as, if not greater than, the annual range; the diurnal variation is much
more marked over land and around the mountains than over the open sea,
since it results chiefly from an insolational increase in vertical convection.
Over the open sea a maximum of clouds (and rainfall) occurs between
4 and 6 A.M., and this undoubtedly also affects such small islands as the
Virgins so that they have in addition to a chief maximum in the afternoon
a secondary one near sunrise.
Except possibly in the rainiest months the high clouds (chiefly strati-
form) are generally much more extensive than (though not so dense as)
lower ones (mostly cumuliform). The cloudiness does not vary greatly
from month to month; the dry months of February and March usually
average between a third and a half of the sky covered; the rainy period of
July to November averages between 0.4 and 0.6 of the sky covered, de-
pending on elevation and exposure of the station. Perfectly clear or en-
tirely overcast days are rare.




SCIENTIFIC SURVEY OF PORTO RICO


The prevailing character of the clouds in these two seasons is apt to be
rather different, however. The rain-season clouds are more often cumulo-
nimbus or cumulus of considerable vertical development with much cirrus,
whereas the clouds of the dry season are smaller fair-weather cumulus
and broken stratiform types (stratocumulus and altocumulus) for the
most part. One's impression is that the sky exhibits a greater variety in
form, density, and arrangement of clouds from season to season than the
observations of "amount of cloudiness" would suggest. The cloudscapes
also reflect certain subtle changes, not yet fully understood, in the general
weather situation; these changes tend to offset some of the monotony of
other aspects of the weather.


STORMS
The characteristics and forecasting of "hurricanes" in this region are
discussed in our report on Porto Rican meteorology; the characteristics
and effects of hurricanes in the Virgin Islands do not differ significantly
from those in Porto Rico and the other Antilles.
The list of hurricanes in appendix B, compiled from many sources, is
probably complete for only the 19th and 20th centuries. Only nine storms
were recorded between 1700 and 1800, whereas twenty-six were noted in
the next century and seventeen so far in the present century. Some small
and weak storms were reported in recent years, but before 1900 many such
storms must have "grazed" the islands without exciting special mention in
the annals. Some of the storms listed were recorded as passing only close
enough to cause heavy rains and moderate gales with but minor, if any,
damage. Somewhat less than thirty hurricanes actually struck one or more
of the islands in full force and with very serious damage since about 1770
(when fairly complete annals begin). The summary in appendix B indi-
cates about 43 per cent of the storms have struck in August, 35 per cent in
September, 18 per cent in July, and the rest in October.
It is not minimizing the danger from these storms to say that they rarely
strike with great severity. The hurricane of August 1772 was described
in a now widely-quoted letter, written by Alexander Hamilton (appendix
B), then a youth of St. Croix, who left the island soon after for North
America. Perhaps the most destructive storm at St. Thomas was that of
October 29, 1867, of which graphic accounts were published in the "Sct.
Thomac Tidende" in early November of that year; a severe earthquake
and tidal wave on November 18 added to the misery. That of September
1876 was also devastating. Of recent years the storms of October 9, 1916,
August 28, 1924, and September 13, 1928 (St. Croix), were especially




STONE: METEOROLOGY OF THE VIRGIN ISLANDS 43

damaging. It should be noted that only about a third of the storms that pass
over St. Thomas pass over St. Croix or cause damage there, and vice
versa. Only twenty of the fifty-one storms since 1738 listed in appendix B
affected both islands, though they had about the same total number of
storms (St. Thomas 39, St. Croix 33). This is easily explained by the
small diameter of the damaging part of most hurricanes and by the various
directions from which they approach the islands (from E through SSE,
approximately). Hurricanes passing to the south of St. Thomas, even
a hundred miles away, can severely damage shipping and docks in the
harbor merely from the heavy southerly swell, though no gale or rain may
be observed; the harbor is protected from swells if the storm passes well
to the north. For details of some of the past storms in the islands see ap-
pendix B and the bibliography.

WIND

Surface Wind Velocity and Direction
We have little representative data concerning wind velocities and di-
rections. The anemometer on the Custom House roof at Charlotte Amalie
apparently has only been used for spot readings, which in the hurricane
season have been cabled or radioed to the U. S. Weather Bureau; means
for several years appear in TEXT TABLE 2.* At Bourne Field a pressure-
tube anemometer has been in operation for several years and the records
are on file there and at the U. S. Navy Department, Bureau of Aero-
nautics, in Washington. Mean velocities cannot be obtained easily from
this kind of instrument but the maximum gust velocities are given in AP-
PENDIX TABLE 16. At Anna's Hope, St. Croix, an anemometer has been
exposed beside the evaporation pan (1 % feet above the ground in a spot
somewhat sheltered by nearby buildings). The record is summarized in
TEXT TABLE 18. These values must be multiplied by at least 4, or 5, to ob-
tain an estimate of the velocity that would be recorded on top of a tower
30 feet above the ground but are representative of a typical microclimate.
It is interesting to note how the wind dies down at night near the ground
at Anna's Hope; the nocturnal radiation forms a shallow layer of cool,
stable air which remains nearly calm in the valley because it is denser and
exerts more frictional resistance than in daytime. The result is a more or
less pronounced diurnal cycle in the wind like that in Porto Rico with high-
est speeds in the afternoon, lowest in the early morning.
See the maps in Tannehill, I. R. "Hurricanes, their nature and history". Princeton, 1938.
No severe hurricane struck the island during this period, though some passed near enough to
cause high winds. The anemometers are usually carried away by a full hurricane hence the highest
velocities are not known; they often exceed 150 miles per hour.
t The anemograms were inspected by the writer at San Juan and Anna's Hope.





44 SCIENTIFIC SURVEY OF PORTO RICO

TEXT TABLE 18
AVERAGE MONTHLY WIND VELOCITY AT THE EVAPORATION PAN*
AT EXPERIMENT STATION, ANNA'S HOPE, ST. CROIX
(From U. S. Weather, Bureau, San Juan)

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year

1920 1.8 1.4 1.9 1.4 1.8 2.2 2.8 2.4 1.8 1.1 1.2 1.7 1.8
1921 2.1 1.7 1.4 1.0 1.3 0.7 1.9
1922 1.2 2.0 2.1 2.0 2.3 2.3 2.5 2.3 2.0 1.1 1.1 1.6 1.9
1923 2.0 2.3 2.4 2.0 2.0 2.6 2.8 2.6 2.0 2.4 0.7 1.9 2.1
1924 2.3 1.8 1.5 1.6 2.1 2.1 2.5 2.2 2.0 0.8 0.9 1.4 1.8
1925 1.5 0.8 1.7 1.3 1.2 1.1 1.0 1.1 0.7 1.2 1.2 1.0 1.2
1926 1.9 1,7 1.3 2.0 1.6 1.7 1.6 1.2 0.9 0.8 0.9 1.5 1.4
1927 1.3 1.3 1.2 1.5 1.4 1.5 2.0 1.0 0.6 0.7 1.2 1.2 1.2
1928 1.5 1.4 1.5 1.6 1.2 1.5 1.6 1.2 2.9 0.8 0.5 1.2 1.2
1929 2.1 1.6 1.8 1.4 1.6 1.5 1.9 1.6 1.0 0.9 1.6 2.0 1.6
1930 1.9 1.0 1.3 1.5 1.4 2.2 2.2 1.8 1.7 1.2 1.5 1.6 1.6
1931 1.3 1.4 1.6 2.0 1.6 1.5 1.7 1.8 1.0 0.4 0.5 1.9 1.4
1932 2.3 1.8 1.8 2.1 1.8 2.0 1.8 1.7 1.3 1.4 1.7 1.7 1.8
1933 1.7 3.0 2.7 1.9 2.2 1.8 2.4 1.5 1.6 1.2 1.5 1.9 1.9
1934 1.5 1.4 2.3 1.9 2.1 1.7 2.1 1.5 1.0 1.1 1.5 2.0 1.7
1935 0.9 1.1 1.8 1.2 1.3 1.3 1.7 1.2 1.4 0.7 0.7 1.2 1.2
1936 1.8 1.4 0.8 1.4 1.0 0.9 1.2 1.0 1.4 0.8 0.5 -
1937 1.2 0.6 0.9 0.9 0.9 1.1 1.1 1.1 0.8 0.6 0.7 1.0 0.9
1938 1,2 1.2 1.2 1.0 0.9 1.0 0.7 0.8 0.5 0.2 0.6 0.4 0.8
Totals 29.3 26.2 29.8 28.7 28.4 30.0 35.7 29.7 26.0 18.4 19.3 26.4 -
Averages
(Mi./
hr.) 1.63 1.41 1.65 1.59 1.57 1.66 1.88 1.56 1.37 0.97 1.07 1.39 1.48

The anemometer is mounted about 1%'/ feet above the ground and hence gives velocities which are
much lower than those read from anemometers as usually exposed on high poles or tops of buildings.

The more exposed, especially windward, parts of the Virgin Islands do
not experience such a locally-produced night calm, though the winds may
die down slightly at night, because at sea the trade wind has a diurnal ve-
locity range of less than 1 mile per hour. Palgrave (infra) thought that the
wind force at St. Thomas often showed a secondary increase between 2
and 6 A.M., thus tending to produce a double maximum for the day (after-
noon and early morning). We have no data from the Virgin Islands to
verify this. Since it is not in accordance with observations from ships over
the tropical seas,* if real it can only be the effect of the diurnal tempera-
ture changes over the island on the stability and frictional stress in lowest
air layers, which normally occurs in more striking form only over larger
islands (see second paragraph infra).
Leeward slopes and shores have some protection from the full force of
the trades, but it is surprising how these winds pass over and around the
islands without much diminution in velocity so that few spots lack a rela-
tively fresh "sea breeze" most of the day whenever the trades attain con-
siderable velocity over the surrounding sea.

Conrad, V. Meteorological results of the "Meteor" expedition 1925-27. Bull. Am. Met. Soc.
23 (4): 143-147. 1942.




STONE: METEOROLOGY OF THE VIRGIN ISLANDS


From day to day the general velocity of the trades often varies decid-
edly; one day it may average 5 miles per hour and the next 15 miles per
hour. Trades in excess of 15 miles per hour of course occur more fre-
quently in the months of highest average velocity, which are in winter and
in mid-summer. Calm spells may be expected during May, June, Septem-
ber, and October. The winds at sea blow as hard from one quarter as an-
other, except in October and November when SE and S winds are usually
lighter than others. The prevailing fresh breezes, coupled with the open
vegetation, dry soil, ample sunshine, and moderate rainfall explain the
reputation of the islands for a relatively healthy and comfortable sub-
tropical climate. The changes in wind velocity from day to day are probably
more important as physiological stimuli than the temperature changes,
which average only 1 or 2' from day to day.
On St. Croix the agriculturalists have found a need for windbreaks,
especially in the flat southwestern section, to protect garden crops and or-
chard fruit trees from the effects of the constant trade winds. The fruit
trees have an unthrifty appearance in unprotected sites on St. Croix and
it is found that such trees suffer from heavy scale infestations. In the lee
of windbreaks of suitably chosen type the scale insects are held in check by
the fungi that thrive better in the more humid conditions provided by the
protection of the windbreak; less spraying of the trees is necessary in the
lee therefore. The Experiment Station has been trying out various plants
for windbreaks. In any case the protective effect does not extend more than
several hundred feet to the leeward. In some parts of the island there is
ample natural protection from woods and hills.
The islands are too small to develop a noticeable system of land and sea
breezes as is found in Porto Rico. Since the wind has some diurnal varia-
tion in velocity and normally speeds up during the morning and falls off
at night, residents often speak of the "sea breeze" arriving in the morning.
There may be a relative calm at night (as at Anna's Hope), especially in
sheltered valleys and on leeward sides, which increases the discomfort
on summer nights, but no true off-shore "land breeze" at night seems pos-
sible. Sometimes shallow layers of radiationally cooled air may be detected
slowly flowing down the hillsides into valley bottoms in the early morning
hours, but this is a very weak and restricted phenomenon here compared
with what it is in the Greater Antilles and in higher latitudes, for it pro-
duces neither a marked local cooling nor a noticeable "breeze" (less than
1 meter per sec.). Usually the trades blow strong enough to mask almost
completely any tendency for land and sea breezes to change the wind di-
rections.
The wind-direction data from Christiansted (TEXT TABLE 19), St.





46 SCIENTIFIC SURVEY OF PORTO RICO

Thomas (TEXT TABLE 2), and other stations (APPENDIX TABLE 11) are not
exactly representative of the general winds because of the interference of
nearby hills and mountains.*


TEXT TABLE 19
PERCENTAGE FREQUENCY OF WIND DIRECTIONS (1875-1907)
CHRISTIANSTED, ST. CROIX (MEAN OF 3 OBSERVATIONS PER DAY)
(From Willaumne-Jantzen)

Direction
N NE E SE S SW W NW Calm

January 3 28 51 9 1 8
February 3 25 47 13 2 1 9
March 5 29 43 10 2 1 2 8
April 3 21 43 18 5 1 9
May 1 10 46 27 7 9
June 10 57 25 3 5
July 20 62 15 1 2
August 1 22 51 16 3 7
September 1 15 42 19 5 1 1 16
October 1 13 36 19 7 1 1 1 21
November 2 25 42 12 2 1 16
December 5 29 45 8 2 1 10
Year 2 21 47 16 3 <1 <1 1 10



Upper Winds
The directions of the lower clouds, as given in APPENDIX TABLE 16 for
Bourne Field and in Palgrave's notes below, are roughly indicative of the
undisturbed trade winds. The pilot-balloon ascents made by the Navy at
Bourne Field have not been summarized. A technical discussion of the
Caribbean upper-air circulation appears elsewhere.t


Winds and Weather Types (after Palgrave)

W. G. Palgrave, a one time authority on the West Indies, observed the
following features during residence at St. Thomas (written in 1874).

"During an average period of nine months in the year the regularity of the air-
currents over the Virgin group resembles clockwork. The surface, or lowest cur-
rent, is formed by the trade wind, which blows briskly from the east-north-east,
with a slight variation northward during the night and early morning, and a cor-
responding deflection southward from noon till near sunset. Its greatest strength is
usually at or near 3-4 a.m.; and about the same hours p.m. It generally bears with it
light masses of cumulus, from which there fall occasionally showers, heavy, but
very short in duration. This air current is known as the trade wind of these regions.

Wind roses for the surrounding waters will be found on the "Pilot Charts of the North Atlan-
tic" (monthly), published by the U. S. Hydrographic Office.
t Stone, R. G. On the mean circulation of the atmosphere over the Caribbean. Bull. Am. Met.
Soc. 23 (1): 4-15. 1942.





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 47

Next above this current comes the south-west wind, rarely absent; it brings with it
light cirrus clouds, but seldom cumulus or other indications of rain. . Highest
of all the west wind reigns, manifested by very light cirrus clouds, rapidly formed
and as rapidly disappearing. These three winds blow with scarcely any interruption
from November to June inclusive; almost the only variation being then afforded by
the north or north-north-east wind which sometimes prevails, but near the surface
only northerer' influence], for a few days together during three winter months.
When- a rare but much desired event [for the crops and water supply] -a south-
erly current occurs about this time, it brings heavy clouds and abundant rain. While
the wind is from the north and north-east [relatively], great dryness is indicated by
the hygrometer. But in the months of August, September, and October, and often in
the latter half of July, the polar or north-east current loses its strength and is often
neutralized or even conquered by the southerly winds. These during the summer are
usually light, and accompanied by a clear and serene sky, only clouded when the
north-east, regaining for a time its supremacy, drives the south back, and precipi-
tates heavy showers, amid thunder and lightning, sometimes lasting for three or
even four hours; after which the wind veers round again to the south-east and
south. The same phenomena, when intensified, concentrate themselves into a hurri-
cane or cyclone a rare occurrence in this island, not more than four of any great
severity having taken place at St. Thomas in the course of the present century
[1800-74].
"Another phenomenon peculiar to the winter and spring months are the white
squalls, which take place on calm days, generally at noon, and most often at no great
distance from shore; their area is very limited, and their duration does not exceed
a few minutes; in some respects they resemble a miniature hurricane, and appear
to be due to similar causes; but neither have I witnessed in them nor heard re-
corded any instance of circular motion. They are much dreaded by the small craft
of these seas; a slight fall of the barometer is their only premonitory indication".
["Black squalls" with dark shower clouds are also feared by the sailors along the
West Indian coasts. R. G. S.]

The above note shows a good elementary comprehension of the wind
system for one writing at such an early date. In general the upper winds
over the Virgin Islands are no different from those over San Juan, Porto
Rico, which have been given special study by Fassig, Ray, and Stone.*
Quin (1907) has published observations of the wind behavior over the
Virgin Islands preceding hurricanes passing the vicinity.


VISIBILITY
The visual range ("visibility") seaward from the U. S. Marine Corps
Base at Bourne Field on St. Thomas is given in APPENDIX TABLE 14. The
visual range is more constantly good than at San Juan, as there is no smoke
or land haze. On specially clear days the mountains of eastern- Porto Rico,
some sixty miles away, can be seen. The factors affecting the visibility are
discussed in Part 3. The range is never less than 2.5 miles except during
heavy rains.
Stone, R. G. Bull. Am. Met. Soc. 23 (1): 4-15. 1942.





48 SCIENTIFIC SURVEY OF PORTO RICO

CLIMATE, DISEASES, AND HEALTH*
We have already mentioned some effects of temperature changes and
winds on comfort and health. Of course, in earlier times the knowledge of
medicine and public hygiene was insufficient to cope with the predisposing
conditions for certain diseases and debility which a warm, moist, monoto-
nous climate offers. It was formerly believed that the climate itself directly
produced tropical diseases, and much was unfairly blamed on the climate
which we now recognize as only indirectly, if at all, related to the climate.
The monotony and mildness tend to lower the resistance of the body, but
better individual and public hygiene can probably offset this danger. A
certain percentage particularly of white people cannot tolerate the climate
well in spite of good hygiene, yet this is true in some degree of any climate.
Constitutions differ due to inherited or acquired idiosyncrasies, and some
are better adapted to one place than another. In another decade we expect
research in tropical physiology will make it possible to select by previous
examination the persons who will adapt well to a tropical climate, and
many of the tragedies that follow assignments to tropical stations will be
largely avoided. On the other hand, it must be recognized that the benefits
of hygiene and medicine are achieved in the tropics at a greater cost of
money and inconvenience than in higher latitudes because the continually
warm moist conditions cause a greater growth of pathogenic organisms
which have to be combatted, than in a climate with a cold winter. It can
be said in favor of the subtropical climate that the tendency to degenera-
tive functional diseases of the heart and to metabolic diseases is distinctly
less than in higher latitudes with their many sharp weather variations that
can overtax the body.
Knox (1852) described conditions which are not much different from
those of today as follows :

". in winter and spring, fever and ague are apt to prevail in the low grounds and
towns; bilious fevers make their attacks more generally in the fall, induced by ex-
posure to rains and the hot sun or intemperance [author's italics]. Consumption car-
ries off many of the inhabitants [as today]. Rheumatism and neuralgia are common.
Dysentery and influenza are epidemic. Whooping cough, scarlet fever, and measles
are almost unknown. The continuous heat of summer and winter ultimately debili-
tates the system and induces disease, especially bowel complaints. Foreigners are
less sensitive to cold in the West Indies than the Creoles, but they feel the heat
more.

Owing to defects in diagnosis, the Danish official statistics of disease
and death are probably in part misleading as to the prevailing ailments.
After 1917 the United States administration has paid more attention to
public health, and conditions are now vastly improved. Some prevalent dis-

Stone, R. G. "Health in tropical climates", in "Climate and Alan". 246-261. U. S. Dept.
Agric., Yearbook for 1941.




STONE: METEOROLOGY OF THE VIRGIN ISLANDS 49

cases were formerly not understood or barely mentioned, such as hook-
worm, venereal diseases, and elephantiasis. The relative dryness of the
islands considerably limits malaria, schistosomiasis, hookworm, and prob-
ably other parasitic diseases. St. John, being heavily wooded and thus hav-
ing moist ground, has more malaria. The swamps, crabholes, and ponds
near the coast, and moist crypts of trees and bushes, as well as cisterns and
ditches, form the foci for local endemism of malaria, elephantiasis, and
other mosquito-borne diseases. An unusually rainy season is followed by
an epidemic increase of malaria (Shaw).
McKinley ("A Geography of Disease" 1935) lists the following as
important diseases in Porto Rico and the Virgin Islands: Malaria, lep-
rosy (a few cases), syphilis, typhoid and paratyphoid, sprue, ascoriasis,
hookworm, schistosomiasis, filariasis, elephantiasis. ulcus tropicum, mal-
nutrition, measles, German measles, varicella, influenza, colds, pneumonia,
tuberculosis, meningitis, diphtheria, nephritis, pellagra, and cardio-vascu-
lar ailments.
It is of interest to note the terrible consequences of a hurricane (see Red
Cross report on 1928 storm) the death rate increases not only because
of injuries at the time, but also from overcrowding and malnutrition dur-
ing the next few months.
The intensity of the sunshine and its richness in ultraviolet have an im-
portant indirect influence on health by drying the soil and killing bacteria
and parasites. The ultraviolet (UV) will give the untanned skin an in-
tense sunburn (erythema) in a fraction of an hour when the sky is clear
and the sun is high. Although severe erythema should be guarded against,
the general exposure to UV produces vitamin D in the skin and helps un-
derprivileged folk to withstand malnutrition and especially rickets.
Heat stroke and sunstroke are rare in the islands because the combina-
tions of wind, temperature, and humidity are seldom so extremely uncom-
fortable as are the hot spells of more continental climates.
Milam and Smillie (1931) studied "colds" and oral bacterial flora of
natives on St. John finding the flora very constant through the year with
but few of the transient flora and seasonal changes found in northern
United States. This is due both to the even climate and the isolation.

CLIMATE AND SUGAR CANE IN ST. CROIX*
Lack of sufficient moisture has always been the major limiting factor to
the sugar industry of St. Croix. For an average cropt at least 45 inches of
Extract with minor changes and some additions by R. G. Stone, from "Geographic Studies on
the Virgin Islands of the United States", by Earl B. Shaw, Ph.D. Thesis, Clark University. pp.
36-58. 1932.
t The average production for plant cane is 17 tons per acre, for cane ratoon, 8 tons; these aver-
ages and rainfall requirements for "good" and "average" crops were furnished by Glen Briggs
(formerly) Director U. S. Agricultural Experiment Station, St. Croix.




50 SCIENTIFIC SURVEY OF PORTO RICO

well-distributed rainfall are necessary, and from 50 to 60 inches are re-
quired for what may be termed a good yield. Rainfall records for 80 years
at Christiansted show that during only 43 years was the annual total 45
inches or more, while during the remaining, the totals range down to 29.48
inches (1873) (see TEXT TABLES 20, 22, and 23). Not only is the yearly
total rainfall unfavorable to sugar production but the distribution through-
out the year is not dependable. There may be a yearly total of 50 inches or
more, enough for a good crop if rightly distributed. But if approximately
one-fourth to one-third of this rainfall comes during one month of the
year, as it often does (see TEXT TABLES 3, 6, and 7), due in most cases to
hurricane influence, and the balance not always well apportioned during
the remaining months, the yearly total has little significance.
Although monthly rainfall is often unequally distributed, the data on
the average number of days with rain (APPENDIX TABLES 3-6, and 12) in-
dicate that the daily distribution may be favorable. In general, this does not
convey the whole truth. Rain may fall frequently throughout the growing
season, but after the cane covers the ground it takes a heavy shower to
reach the roots and to benefit the plant. Such showers come too infre-
quently for ideal crop development, as St. Croix is too small and too low
to bring about marked local heat-convcctional or orographic rainfall.
Wind is another climatic hazard to sugar cane in St. Croix. Tropical
hurricanes may come anytime from June through November and some-
times damage a growing cane crop or ruin it entirely (see appendix B).
These storms not only cause loss of cane, but also inflict serious injury to
buildings and the laborers. They are regarded with such dread by the
people that special prayers for deliverance are made at the beginning of the
hurricane season, and a thanksgiving service is held when the period of
danger is past. The constant trade winds also cause a high rate of evapora-
tion which is unfavorable to soil-moisture conservation. The mean annual
evaporation from a water-filled pan over a period of years (1920-38)
shows that the potential evaporation exceeds the mean annual rainfall by
about 27 inches (see APPENDIX TABLE 8); but the actual evaporation from
the land and vegetation is not as great as from a water surface represented
by the pan. .
On the other hand, temperatures and sunshine are favorable for sugar
production, low latitude and altitude eliminate frost danger, and the large
percentage of possible sunshine favors a high sucrose content (see TEXT
TABLES 1, 17, and APPENDIX TABLES 9-12, 15).
The rainfall map of St. Croix (FIGURE 2) was made from data fur-
nished by the Agricultural Experiment Station at Anna's Hope. Rainfall
records (APPENDIX TABLE 7) from 26 stations (mostly sugar estates), in-





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 51


eluding Christiansted and Fredericksted, for a period of ten years were
used in drawing the isohyets. Only one station (Cotton Grove) with but
one year's precipitation records was available for the far eastern section,
but it is known that eastern St. Croix has a considerably lower rainfall than
other parts of the island (see discussion of Vegetation and Climate).
Northwestern St. Croix is the rainiest section, and the south has less
precipitation. The trades (prevailing direction ENE-ESE) blowing over
the northwest upland account for the greater precipitation there. In eastern


TEXT TABLE 20

SUGAR-CANE YIELD AND RAINFALL OF ST. CROIX, 1862-1938


Years 1862-1902
(From St. Croix Gov't: "Statistics
concerning sugar production . .")


Yield
Year Acres *hogshead/ Rainfallt
fiscal taxed acre "lines"

1862 18,074 0.72 330
1863 17,535 0.56 272
1864 17,449 0.38 316
1865 17,602 0.47 396
1866 17,475 0.73 371
1867 17,505 0.60 373
1868 17,326 0.64 302
1869 17,276 0.37 382
1870 17,276 0.54 410
1871 17,277 0.99 287
1872 17,221 0.40 255
1873 17,137 0.40 237
1874 17,089 0.18 385
1875 16,835 0.61 245
1876 16,554 0.32 286
1877 16,608 0.30 391
1878 16,576 0.62 454
1879 16,574 0.53 541
1880 15,664 0.64 331
1881 15,980 0.62 464
1882 15,963 1.02 354
1883 16,486 0.68 386
1884 16,700 0.94 356
1885 16,508 0.91 365
1886 16,548 0.80 433
1887 16,511 0.90 383
1888 16,440 0.81 438
1889 16,479 1.16 437
1890 16,489 0.98 243
1891 16,333 0.21 403
1892 16,404 0.94 282
1893 16,587 0.62 448
1894 16,633 1.22 352
1895 16,011 0.85 446
1896 15,974 1.15 448
1897 16,149 1.21 439
1898 16,181 1.04 404
1899 16,187 1.28 273
1900 16,298 0.70 364
1901 16,441 1.23 535
1902 16,428 1.29 491


Years 1900-1938
(From U. S. Dept. Interior: "The
Virgin Islands of the U. S.", 1935)


Yield Rain-
Acres short fall
taxed tons in.

16,298 8,614 45
16,441 15,111 67
16,428 15,937 61
15,820 9,419 45
15,704 11,231 37
15,194 4,978 53
15,068 8,541 56
13,986 7,940 38
13,550 4,859 45
14,007 6,444 52
13,901 7,516 43
13,710 7,209 45.5
13,397 4,831 37
12,744 4,203 39
11,898 3,653 37.5
12,474 3,037 65.5
12,220 15,334 59
12,627 7,725 39
12,718 5,841 46
12,498 9,723 51.5
12,847 13,329 34.5
11,854 30
9,662 6,345 26.5
9,014 1,948 33
9,208 2,385 39
9,585 10,653 52
9,196 6,343 39
9,250 6,860 48.4
8,240 11,275 43
8,135 2,825 60
5,892 39
5,009 1,787 49
4,686 4,287 70
4,505 4,125 68
5,386 4,088 53
5,800 1,670 40
6,277 3,725 68
5,266 5,749 44
6,112 4,664 43


* Ilogshead = 1,500 lbs. f Averages of 2 or 3 stations, as in APPENDIX TABLE 3 and TEXT TABLE 5.





52 SCIENTIFIC SURVEY OF PORTO RICO


St. Croix, the elevations are insufficient to produce much orographic rain-
fall and the windward situation and narrow land cause fewer and less in-
tense convection showers from local insolation than the wider leeward
western portions.
The TEXT TABLES 20, 21, and 22 show that the years of heavier sugar
production generally follow those of increased rainfall. This is not always


TEXT TABLE 21

DEPARTURES OF RAINFALL AND SUGAR YIELD IN EXTREME YEARS FROM THEIR
NORMALS FOR 1881-1904 AND 1882-1905, RESPECTIVELY, IN ST. CROIX

(Mean rainfall was 1242 mm.; mean sugar yield 1202 lbs. per acre)
(From Ravn)


A. Wet Years


B. Dry Years


Annual April-November


Rain-
fall de-
Year parture
%


Sugar
yield
departure
(next yr.)
%


No. days
with
20 mm.
Rain- or
fall de- more
parture rain
%


881 +19 + 8 +31 -
886 +11 4 + 6 16
888 +12 +26 +15 11
889 +12 + 4 +18 22
893 +14 +28 +19 11
895 +14 +25 + 5 14
896 +14 +32 +18 15
897 +12 + 9 +14 17
901 +37 +40 +44 17
902 +25 6 +22 -


Annual April-November

No. days
with
Sugar 20 mm.
Rain- yield Rain- or
fall de- departure fall de- more
Year parture (next yr.) parture rain
% % %

1882 -15 -26 -18 10
1890 -38 -76 -54 3
1892 -28 -30 -24 7
1899 -31 -24 -26
1904 -24 -49 -31 5

Mean -27 -41 -31 6


Mean +17 +16 +19 15



TEXT TABLE 22

FREQUENCY OF ANNUAL RAINFALL TOTALS, 1852-1914 (AVERAGE OF 3 STATIONS),
AND CORRELATED SUGAR YIELDS, 1878-1915, ON ST. CROIX

(From Ravn)


Sugar yield


Number of
cases

6
20
27
8
2


Tons of cane
per acre
(1878-1915)

4.7
7.0
9.7
10.6
12.2


Hogsheads of sugar
per acre

37
58
87
98
(97)


Class intervals
Inches of rainfall

26-35
35-45
45-55
55-65
65-70





STONE: METEOROLOGY OF THE I'IRGIN ISLANI)DS


the case, for sometimes a large percentage of ratoon cane and an epidemic
of plant diseases or insect pests cause a low yield even after years of heavy
precipitation. The distribution of rainfall through the year is equally as
important as the mean annual rainfall. The data illustrate the erratic na-
ture of the rainfall, a characteristic much emphasized in discussions of
some of the temperate regions but not always stressed sufficiently for parts
of the tropics. The area of production showed a gradual decline from ap-
proximately 20,000 acres in 1850 to about 12,000 in 1900. Since then the
acreage has decreased rapidly until in 1930 it dropped below 6,000.


TEXT TABLE 23
RAINFALL OF THE FIVE WETTEST AND FIVE DRIEST YEARS, ST. CROIX,
1852-1914, IN INCIIES (AVERAGE OF 3 STATIONS)
(From Ravn)

Wettest Driest
1861 58.8 1863 34.0
1879 67.6 1872 32.0
1881 58.0 1873 29.6
1901 66.9 1875 30.6
1902 61.4 1890 30.3


THE UPPER AIR AND GENERAL CIRCULATION IN RE-
LATION TO THE WEATHER AND CLIMATE
Aerological observations have been made at Bourne Field by the U. S.
Marine Corps since 1935, airplane-meteorograph soundings being included
since January 1937. The pilot-balloon ascents have reached only several
kilometers on the average and have not been summarized. A long series of
observations of winds aloft at San Juan, discussed elsewhere,* is suffi-
ciently representative of conditions over the Virgin Islands, except for
the lowest 500 meters.
Some of the most general features of the circulation are mentioned in
the note by Palgrave above. More specifically, the troposphere in this re-
gion stratifies itself into two primary layers which form two closely-related
fundamental branches of the general circulation of the whole earth's at-
mosphere, viz., the trades, an easterly current at the surface, and the anti-
trades, a westerly current flowing above the trades. Both currents are
present throughout the year but they vary seasonally in depth, velocity, di-
rection, and steadiness. The average height of the center of the transition
zone between the top of the trades and the bottom of the antitrades ranges

Stone, R. G. Bull. Am. Met. Soc. 23 (1): 4-15. 1942.




SCIENTIFIC SURVEY OF PORTO RICO


here from about 4.5 kilometers in winter to 10 or 12 kilometers in summer.
The transition is rarely sharp but may be from tens or hundreds of meters
to several kilometers thick; also it varies rapidly in height from day to day
about its mean height. Rarely the westerly winds may come down to the
very surface, as in the front of approaching hurricanes or with northerss".
The trades blow rather constantly from slightly north of, or due, east in
winter and from somewhat south of east in summer (see TEXT TABLES 2
and 3) ; but at the surface other directions appear more frequently than
aloft due to the deflection by hills and the effects of showers and squalls,
and possibly to land and sea breeze tendencies. The friction of the winds
with the earth's surface causes a regular logarithmic diminution in the
speed from about five hundred meters down to the ground, and also causes
a slight turning of the direction more to the left (looking downwind)
which means the surface wind comes from more south of east than aloft,
on the average. All wind observations for the lower atmosphere, whether
from cloud motions, balloons, or wind vanes and anemometers, must be
interpreted with these facts in mind.
APPENDIX TABLE 15 gives a summary of mean resultant motions of
lower, middle, and upper-level clouds at 3 observations a day at Bourne
Field. The results show the general prevalence of trades and antitrades
but there is much variation from year to year and month to month which
is not to be taken as entirely real since the clouds are apt to be associated
more often with certain directions than others. The monthly resultants
from regular daily pilot-balloon ascents at San Juan do not show such large
variations as do these cloud observations, and the former are for various
reasons probably more representative of the true conditions.
APPENDIX TABLES 17-20, containing the average temperatures, pres-
sures, and humidities at fixed heights up to 5000 meters, are based on air-
plane-meteorograph soundings made daily during 1937 and 1938 at
Bourne Field. In 1939, 1940, and 1941 soundings were resumed during
the hurricane season (July-November) and on special occasions (maneu-
vers). The interpretation of individual soundings and their significance
for various problems in tropical meteorology would be disproportionately
long and out of place here; but there are several features of the prevailing
thermodynamic structure of the free air which should be described briefly
because they determine to a large extent the rainfall regime of the islands.
A perusal of the mean values of the upper-air humidities shows that the
specific humidity is considerably higher in the summer (rainy) season than
in winter both at the surface and upper levels; but the relative humidity
variation differs with height, for while aloft it is much higher in summer
than winter, in the lowest 2.5 kilometers it is rather uniformly high through





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 55


4ooo 3ooo



SZooo ---JAN oo ... APR
-- FES MAY
ooo ---- MAR oo --- JUNC

-o0 0 /0 20 30 P -10 0 /0 20 30
DECr. C DEG C
,ooo I, ooo \.




Zooo JULY Zooo ---- OCT
S AUG N NOV
e ooo .P \1- "-looo DEC


-/O C Y o ZO 30 -lo 0 /0 2o0 3
DEG. C DEG. C
FIGURE 3. Average lapse rates over St. Thomas, 1937-38. Temperature in C. plotted against
height in meters. One dry and one wet adiabat are added for reference, originating at 20 and
30* C. at the surface, respectively.

the year. This distribution of relative humidity is graphed in FIGURE 9.
Now recalling the annual course of monthly average rainfall totals at sta-
tions in the islands (APPENDIX TABLES 3-7), it is evident that the "rainy
season" from May through November coincides with the period during
which the humidity in the levels above 3 kilometers is high. Such a relation
is not surprising from what is generally understood or assumed about the
nature of tropical rainfall processes, for the more active the convection and
the further upward it reaches the greater rainfall that may result, other
things being equal. The convection both carries moisture aloft with it and
in turn is able to penetrate higher by virtue of the resulting increase in
humidity (the condensation of which converts latent into kinetic energy).
It remains to consider the reason for an increase of convective vigor in
the so-called rainy season. A corresponding change in the vertical stability
of the air column with season is indicated a priori, and readily verified by
an examination* of both the mean monthly lapse rates (vertical tempera-
ture gradients) and the lapse rates day by day through the year. FIGURES
3, 4, and 5 show the mean monthly lapse rates and two soundings of con-
trasted types.

The adiabatic charts for each sounding were inspected at Bourne Field and at the U. S.
Weather Bureau, Washington.





SCIENTIFIC SURVEY OF PORTO RICO


FIGURE 4. Upper-air sounding at St. Thomas, March 23, 1938. Plotted on an emagram (co-
ordinates: temperature and logarithm of pressure). The dashed lines are two wet adiabats and
the sloping full straight line is a dry adiabat. The right hand curve of the sounding is tempera-
ture with specific humidity values written beside the significant points. The left hand curve of
the sounding is the wet-bulb temperature, with relative humidity values added for each signifi-
cant point. The range of pressure from bottom to top of the chart is from 1030 to 500 millibars.
This sounding reached to about 18,000 feet.


FIGURE 5. Upper-air sounding at St. Thomas, Nov. 5, 1940. Plotted similarly to FIGURE 4-
This sounding reached about 14,000 feet.




STONE: METEOROLOGY OF THE VIRGIN ISLANDS 57

The soundings made at St. Thomas were the first extensive series from
this part of the sub-tropical Atlantic. From them the writer has noted that
the structure of the trades in the Caribbean is similar to that found years
ago from soundings in the eastern Atlantic off the Canary Islands and
Cape Verde. This structure explains why the whole region of the sub-
tropical North Atlantic (or "Azores") anticyclone has relatively little
cloudiness and rain, for the moist unstable surface air layers in which the
trade-wind cumulus-clouds form are constantly overlaid by a deep warm,
dry, stable layer (or layers) which generally prevents any convection
originating in the surface layers from reaching high enough to produce
heavy showers.
The formation and maintenance of this upper stable layer are not yet
fully understood, except that it undoubtedly has its immediate origin in the
subsidence of the antitrades into the core of the anticyclone. The surface
moist current is separated from the upper dry current by an inversion of
the temperature lapse-rate, or by at least a zone of isothermal lapse rate
or of greater stability than below and above. This is called the trade in-
version. Presumably in many cases the so-called Ts, or S, inversion often
noted above tropical Atlantic air-mass invasions into the United States
were originally trade inversions. The St. Thomas soundings show the trade
inversion in some degree during most of the year. This is not surprising
since the Virgin Islands are nearly always under the southwestern margin
of the Azores anticyclone, but what it probably really means is that the
inversion is being constantly carried along outwards from where it is
formed in the central and eastern parts of the anticyclone. A comparison of
soundings made in different parts of the anticyclone shows that the height
of the base of the inversion tends to rise in all directions from the center of
the anticyclone.* Thus near the Cape Verde Is. it is only about 500 meters
above sea level on the average but increases to 1800 meters near the equa-
tor and over the Caribbean. The stability, dryness, and thickness of the up-
per current tend to decrease as the height of the inversion increases, and
vice versa.
When the daily soundings at a fixed station, such as St. Thomas, are
compared successively it is immediately apparent that the inversion and the
air above it vary remarkably in height, thickness, intensity, and other char-
acteristics from day to day. Furthermore a closer analysis leaves an im-
pression that the day to day changes in cloudiness, rain, and wind are
somehow related to the changes in the stability of the air aloft. But to date
it has not been possible to find a simple or clear-cut correlation, owing to
the complications introduced by local convection and surface influences.
von Ficker, II. Die Passatinversion. Verbffentl. Meteorol. Inst. Univ. Berlin 1, Heft 4, 1936.





58 SCIENTIFIC SURVEY OF PORTO RICO


_______JULY __AUGUST SEPT.
5 10 1 5 20 25 3 30 5 IS 20 20 25 3 0 5 I0





BVROME CURVE CURVE
30.I I I \ 1 I" l
S B E RONARON; CURVE ,
1., 2 i V 4 i




3 1 I1,1 i l I1 1I 1 1 I 1 1 1 I 1 1 1 1 I l_1 I






St. Thomas. (After D i n, Bull. Am. Met. Soc. p. 225, June 1940.) '


interested in further details It appears from daily North Atlantic _
the inversion (Ts Inversion Curve) is low. Dunn found that in the July
























change and that in each case the pressurefall phase of the "wave" was as-
W SEPTEMBER OCTOBER NOVEMBER































of specific humidity and temperature at the 3000-meter level. Hurricanes
29on p. 218. 1940.
FIG BUR 6. Surface pressure and height of the trade (= "Ts") inversion, July-November 1938, at
St. Thomas. (After Dunn, Bull. Am. Met. Soc. p. 225, June 1940.)


and to inadequate data.* The attempts of Frolov and of Dunn to find
clues to this problem through isallobaric analysis should be studied by those
interested in further details.t It appears from daily North Atlantic
weather maps that the Azores anticyclone fluctuates from day to day in
size, intensity (maximum pressure), and in position, which in turn causes
some of the day to day changes in the trade inversion at St. Thomas men-
tioned above. Note in FIGURE 6, from Dunn, how the surface pressure
(Barometer Curve) has a rough tendency to be high when the height of
the inversion (Ts Inversion Curve) is low. Dunn found that in the July
period (shown on this figure) there were eight major "waves" of pressure
change and that in each case the pressure-fall phase of the "wave" was as-
sociated with a lifting of the inversion and usually also with an increase
of specific humidity and temperature at the 3000-meter level. Hurricanes

See ref. to correlation coefficients computed by Bice, Bull. Am. Met. Soc. 21: 219, also fig. 2
t Bull. Am. fet. Soc. 21: 216-229. 1940, and 22: 198-210. 1941.





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 59

are observed to develop through continued "deepening" of a pressure-fall
of this type. Of course, the great majority of these falls do not become
hurricanes but remain of small amplitude (.01 to .11 inch) or die out.
J. E. Miller and G. Emmons have investigated this phenomenon further.*
All these pressure changes studied by Dunn were found by plotting maps
of the 24-hour pressure differences which show 48-hour periodicities. The
areas of rise and fall on these maps tend to move from east to west at 15


JULY AUGUST
5 10 15 20 25 50 5 10S 5 20 25 50
ir lll I I ill I I l I 1 1 r r I II i Il Illi Ill
SURFACE PRESSURE IN INCHES





SPECIFIC HUPMIITY IN /K AT 500 METERS *
) ,. ff /A N, /Y


N5./. L -CAT.3000 METERS
*y- ... .... I , _T RE' ', !N., ,, ,*., , " ,5 -
FIGURE 7. Surface pressure, upper-air temperature, and specific humidity at
3000 meters, July-August 1938, St. Thomas. (After Dunn, Bull. Am. Met. Soc.
p. 218, June 1940.)

to 25 miles per hour. However, Frolov has studied the 72-hour pressure
differences which show six-day periodicities. These do not propagate from
east to west but rise and fall in unison at all stations affected by the south-
ern half of the Azores anticyclone from Africa to Central America. This
fact can only mean that these longer fluctuations are produced by expan-
sion and contraction of the entire Azores anticyclone. The shorter-period
changes that propagate are superimposed on these longer periods. No cor-
relation of Frolov's 6-day period in pressure with variations in the other
elements has been made. An inspection of the daily rainfall tables in the
"Climatological Data: West Indies and Caribbean Service" suggests that
the chief variation of rainfall within a month has a rough period of about
a week to 15 days, as a rule. This corresponds to the largest "see-saws" in
pressure shown on one of Dunn's figures (FIGURE 7). Apparently these
U. S. Dept. of Commerce, Weather Bureau, "Curso para el Estudio de los Huracanes, Tomo I:
Caracteristicas del Tiempo en el Caribe y Metodo de Analisis". Mimeo., Wash., D. C., pp. 65-84.
Feb. 1942.
J. E. Miller, "The Significance of Allobaric Systems in Forecasting Cyclogenesis in the Carib-
bean". MS Thesis. College of Eng., New York Univ. Oct. 1941.





SCIENTIFIC SURVEY OF PORTO RICO


grosser fluctuations are due to shifts in the position of the Azores anti-
cyclone in combination with changes in its size and maximum pressure.
But they can be effectively studied only with the aid of weather maps of the
whole northern hemisphere, because they are actually a direct manifesta-
tion of changes in the intensity of the general circulation of the earth's
atmosphere.*
The east-to-west moving allobaric systems are apparently prevalent only
in the "rainy season" (June-November). In the winter season they move
from west to east. The winter systems are of different origin and character
than the summer ones. The latter form closed circular areas whereas the
former are long troughs that extend south or southwestward from cy-
clones of middle latitudes well into the tropics if not often to the very edge
of the doldrums. These troughs are originally associated with the cold
fronts of polar-air outbreaks across the United States from Canada or the
Pacific; the warm Caribbean waters quickly efface any observable tem-
perature discontinuity at the surface before the polar air reaches the Vir-
gin Islands, except in the relatively rare cases of northerss." The anti-
cyclones that follow the cold fronts often continue southeastward to merge
with the west or north side of the Azores High ; such anticyclones are al-
ready largely transformed from cold-air domes to warm-air dynamic anti-
cyclones by the time they reach the subtropics. The airflow through the
preceding trough and the warm ocean surface are then unfavorable to the
continued existence of a temperature discontinuity, but the trough remains
as a col between the newly developed dynamic anticyclone and the pre-
existing one represented by the Azores High. The younger anticyclone does
not simply amalgamate with the Azores High but often appears to "grow"
and to move eastward at the expense of the older High. In this way the
trough between them appears to move eastward across the Atlantic Ocean.
A connection of this trough with the northern part of the cold front from
which it originally derived may often be maintained during the eastward
progress across the ocean, or else with a new secondary (or wave) cy-
clone which may have formed on the front when it was off the eastern
coast of America. In any case the passage of one of these troughs over
the Virgin Islands and the other Antilles usually leads to a noticeable se-
quence of changes in the weather : before the trough passes, a slight shift
of winds toward the south with increasing warmth, moisture and clouds;
Cf. Papers in Physical Oceanography and Meteorology. Mass. Inst. Techn. and Woods Hole
Oceanogr. Inst. 8, no. 3. 1940; and 9, no. 1. 1941. Rossby indicates that the Azores High shifts,
north and south with changes of intensity in the zonal circulation of the westerlies (Jn. Mar. Res.
2 (1): 3S-55. 1939). Clayton correlates these shifts with sunspots (Trans. Am. Geophys. Un. 1941
(II): 420-423).





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 61

then as it passes, a shift towards the north with more rain and cooler air;
after which, clearing and drier weather, gradually reverting to average
trade conditions. The troughs tend to pass this region about once every 5
to 7 days. and thus the fluctuations which they cause may be considered as
part of the "normal" picture for the season. The late Dr. 0. L. Fassig in
1909 began to notice the "influence of passing highs and lows to the north"
on weather in Porto Rico, but lie never published his manuscript notes on
the subject.* Only recently has the phenomenon again come to the atten-
tion of American meteorologists." The accompanying changes in the
upper air and the mechanism by which the trough affects the clouds and
rainfall have not yet been fully investigated. However, it appears that the
belts of convergence and divergence produced by a sinusoidal field of flow,
as demonstrated by J. Bjerknes,+ could account for all or part of the
weather changes by raising and lowering the trade inversion. In addition,
the southerly component in the forward side of the trough might import
moister air aloft from lower latitudes. The intensity and frequency of
troughs should be directly dependent on the character of the meridional
interchange of polar and tropical air masses over eastern North America
- indeed, there is some evidence that during colder winters in eastern
United States the weather is relatively cool, rainy, and cloudy in the An-
tilles.
The origin of the allobaric systems of summer is surmised to be in Af-
rica; from May through November systems of similar character parade
from cast to west across West Africa and out to sea past the Cape Verde
Islands. Violent squalls ("tornadoes") are associated with the katallo-
baric centers. Although the ship observations are too few to definitely
trace the systems eastward to the Antilles, many meteorologists believe
that at least they may occasionally develop into hurricanes upon approach-
ing the Antilles. Hubert indicates a high probability that the great "New
England Hurricane" of 1938 had such a history. But the "tornadoes" are
always observed to weaken east of the Cape Verde Islands and therefore
some process must favor their rejuvenation in the western Atlantic re-
gion.|[

These were edited by us and will appear in Part 3 of this volume.
t See U. S. Dept. of Commerce, Weather Bureau, "Curso para el Estudio de los Huracanes",
Tomo I: 57-62. The role of these troughs in the weather of the Canary Islands region was first
recognized by Roschkott in "Festschrift der Zentralanst. f. Met. u. Geodynamik in Wien, zur
Feier ihres 75 jiihr. Best." Vienna. 1926, p. 121 if. See also Piersig, W., Schwankung Luftdruck
und Luftbewegungen. Archiv d. Deut. Seewarte. 54 (6): 1936.
t Bjerknes, J., Theorie der aussentropischen Zyklonenbildung. Met. Zeit. 54 (12): 462-466.
1937. Lt. Geo. P. Cressman, of New York University, has applied Bjerknes' principle to the
winter troughs of the Azores High. in an unpublished manuscript, 1942.
S Abstract by C. F. Brooks, in Trans. Am. Geophys. Un. 1940 (II): 251-253; also critique by
Portig, Ann. d. Hydrographie, 67 (7): 398-400. 1939.
|| Regula. H. Druckschwankungen und Tornadoes an der Westkuste von Afrika. Ann. d.
-lydrogr., 64: 107-111. 1936; also Piersig, W., op. cit.





62 SCIENTIFIC SURVEY OF PORTO RICO

The occurrence of east-to-west allobaric centers and of West African
"tornadoes" is limited to the period when the boundary between the trades
of the north and south hemispheres is well north of the equator; it is there-
fore a reasonable assumption that the Coriolis force is essential for the
maintenance of these allobaric systems, though they apparently arise
merely from the large diurnal period in temperature, pressure, and wind
over subtropical Africa. However, we recognize the possibility that the
allobaric systems observed in the Antilles may also have a more immediate
origin over the ocean to the southeast due to surges of southern hemi-
sphere trades pushing into the northern trades; such surges are observed
in the doldrums but it is generally believed that many of them are in turn
consequences of the "tornadoes" from West Africa. 1
The subdivision of the subtropical high-pressure belts into cells can per-
haps be better understood in physical terms as a result of lateral mixing.
Rossby and his collaborators* have shown that above the layer of surface
influences the subtropical highs break up into a train of large anticyclonic
eddies, in a manner called for by the assumption of a jet stream suffering
lateral mixing with its environment on a rotating earth. In particular an
analysis by Simmers* of a case of warm anticyclogenesis suggests that the
lateral-mixing process may explain how the separate highs maintain their
identity as they pass across the subtropical Atlantic in winter. In summer,
too, anticyclonic eddies prevail aloft over the United States and Gulf of
Mexico in the seasonal extension of the Atlantic high pressure belt north-
ward and westward.* No studies have been made as yet to discover
whether the weak summer allobaric systems are associated with observable
eddy tendencies aloft.f We offer the tentative view that the winter cells
and troughs of the Azores High are the result of jets of polar air driven
into the westerlies, and the summer allobaric systems are the result of jets
of equatorial or southwest-monsoon air driven into the trades. The much

$ Perhaps in a manner analogous to the wedges and troughs in the westerlies as demonstrated by
Rossby, C. G. Papers in Phys. Ocean. and Met., M.I.T. and W.H.O.I., 8 No. 3: 1940.
Regula, op. cit.; Hubert, H. Grains orageux et les pluies en Afrique Occidentale. Paris, 1922;
id: Nouvelles 6tudes sur la m6tiorologie de 1'Afrique Occidentale Frangaise. Paris, 1926; Fro-
lov, S. Variations de la pression en A.O.F. Ann. de phys. du Globe de France d'Outre-mer, 3
(14): 46-50, 57-58. 1936; Weisse, L. and Barberon, J. Note au sujet du grain du 28 au 30 Juin
1933. etudes met. sur I'Afrique Occidentale Fr., Publ. Comm. d'ttudes Hist. Sci. A.O.F., Sir.
B No. 3: 47-61. Paris, 1937. Goualt, J. Vents en altitude & Fort Lamy (Tchad). Ann. Phys. du
Globe Fr. d'Outre-mer, 5: 90-91. 1938; Farquharson, J. S. The diurnal variation of wind over
tropical Africa. Quart. Jour. Roy. Met. Soc. 65: 165-184. 1939.
|| Ferraz, J. de S. Hurricanes and South Atlantic circulation. Bull. Am. Met. Soc., 20 (8):
334-335, 1939; also Regula, H. Schwankungen der Passatgrenzen. Ann. d. Hydrogr. 65 (10):
458-460. 1937; Durst, C. S. The doldrums of the Atlantic. Geophys. Mem. (London, Met. Off.)
28: 1926.
Rossby, C. G. and collaborators. Fluid mechanics applied to the study of atmospheric circula.
tions, Papers in Phys. Ocean. and Met., 7 no. 1, 1938. See also Namias, J. Isentropic analysis, in
"An introduction to the study of air mass and isentropic analysis". 5th ed., Am. Met. Soc., 1940, pp.
136-161.
t Note, however, the isentropic charts for the hurricane of 1938 by Pierce in U. S. Mon. Wea.
Rev. 66: 237-285. 1938, and the mean monthly isentropic charts published regularly in the same
journal since 1939.




STONE: METEOROLOGY) OF THE 'IRGIN ISLANDS 63

larger, more intense development of the winter systems compared to the
summer ones reflects the difference in latitude and hence in the respective
Coriolis effects.
The mean conditions of a month at a station such as St. Thomas are thus
the result of a complex of fluctuations of various types and causes. The
rainfall is increased by any process which weakens or lifts the inversion,
whether it be simple heating from the surface, convergence from a passing
pressure-change system, or a shift of the Azores anticyclone northward so
that the lesser stability normal to regions nearer the equator spreads to
somewhat higher latitudes. The annual march of rainfall in the West In-
dies can be explained for the most part by seasonal migrations of the
Azores anticyclone (dry) and of the equatorial Low (rainy). The ef-
fect of this migration on the inversion over the Virgin Islands is thus a
regular weakening from winter into the rainy season (May-November),
with a brief partial return to winter-like conditions in July, and a rather
rapid intensification of the inversion from November to January. FIGURE 4
is an example of the temperature-height curve when the trade inversion is
very marked over the Virgin Islands region; the opposite extreme, in
which very little stability is present, is exemplified in FIGURE 5. The more
typical and frequent conditions are of course intermediate between these
rather extreme types.
In APPENDIX TABLES 17 to 20 are averaged together the 1937 and 1938
soundings month by month and level by level for temperature (' C.). pres-
sure (millibars), and relative humidity (per cent), and for 1937 we have
copied the monthly mean specific humidities and equivalent-potential tem-
peratures as published. The levels for which the data are given are: sur-
face, 500 m, 1000 m. 1500 m, 2000 m, 2500 m, 3000 m. 4000 m, and
5000 m.
The mean temperature-height curves are plotted in FIGURE 3 for each
month. A wet adiabat is drawn to the right and a dry adiabat to the left on
each diagram. The general prevalence of conditional equilibrium (lapse
rate between dry and wet adiabatic, i.e., air is stable if unsaturated, but
unstable if saturated, as when a cloud has formed) is very striking. Also
the great stability, approaching inversion conditions, in the layer from
1000 to 3000 meters is prominent from December to April. The trade in-
version is present on so many days that there is almost an inversion in the
averages, in spite of the varying height of the inversion and the large in-
tervals between the standard levels for which the averages are computed.
In order to get a clearer picture of the stratification of thermal stability
and instability, we have read off the lapse-rate types between each level for
each month and tabulated them as follows:





64 SCIENTIFIC SURVEY OF PORTO RICO

TYPES OF MEAN LAPSE RATE ABOVE ST. THOMAS (1937-38)
C stands for conditional equilibrium; W for approximate wet adiabatic
equilibrium; S for absolute stability (approaching isothermalcy or in-
version) ; and U for absolute instability (dry adiabatic equilibrium)
(limits given in meters above sea level)

January February March
C to 1500 m C to 1500 m C to 1500 m
W 1500-2000 m W 1500-2000 m W 1500-2000 m
S 2000-2500 m S 2000-3000 m S 2000-3000 m
W 2500-3000 m C 3000-5000 m W 3000-5000 m
C 3000-5000 m
April May June
C to 1500 U to 500 U to 1000
W 1500-2000 C 500-1500 C 1000-1500
S 2000-2500 W 1500-2000 W 1500-2000
W 2500-4000 C 2000-5000 S 2000-2500
C 4000-5000 C 2500-5000
July August September
C to 1500 U to 500 C to 5000
W 1500-2500 C 500-1500
C 2500-4000 W 1500-2500 December
W 4000-5000 C 2500-5000 C to 1500
S 1500-2500
October November C 2500-4000
C to 5000 C to 5000 S 4000-5000


From this table a cross-section, FIGURE 8, has been drawn in which the
layers or zones of each lapse-rate type are separated by lines, smoothed
somewhat to give a more simple schematic appearance and to interpret in a
logical way some of the improbable angularities imposed on the data by the
limitation of averaging at only a few fixed levels. Various interpretations
could be given in this smoothing procedure, depending on the interpreter's
preconception of the most probable scheme and on other information that
may be available indirectly to corroborate or guide him, assuming of course
that the observations were accurately made and reduced in the first place
and that the number of observations is sufficient to give normal or typical
values.
The writer believes that the data for these two years indicate a nearly
normal state of the upper air in the region in spite of the short record be-
cause the conditions in this tropical maritime region are known to have a
relatively small variability. More data would alter details but not the major
features. An inspection of the individual soundings served to confirm the
interpretation of the mean conditions given in FIGURE 8.
The vertical arrows in FIGURE 8 indicate the relative intensities of up-
ward convection and resistance thereto from stability aloft (downward
arrows). When FIGURE 10, showing the rainfall totals and intensities and
temperatures at Bourne Field for the same years as the soundings cover, is
compared with FIGURE 8 and FIGURE 9 the close relation of the mean rain-
fall to the mean upper-air conditions becomes evident.















FPc-RE 8. Scheme of upper-air structure over the Virgin
Islands. Months plntted against height in meters. S = Stable,
W = Wet Adiahatic, C = Conditional, U = Unstable. Arrows
pointed up indicate by their length the relative intensity of
convection and downward armws the relative resistance to con-
vection.


DJFMAMJJ ASO NDJ


FICUne 9. Mean relative humidity in the upper air over the
Virgin Islands, 1937-38. (Months plotted against height in kilo-
meters.)


FicGUE 10. Monthly meao rainfalls, mean rainfall intensities,
in inches per rain day and per rain hour, and mean maximum
and minimum temperatures at Bourne Field, St. Thomas,
1937-38.


2'

S AIN INT ENSA BY





I Pain Ises me' ,/no ,


1j ToTALr









D J F M AM'J J A S O N D J




STONE: METEOROLOGY OF THE VIRGIN ISLANDS 65

The two chief rainfall maxima, in May-June and October-November,
characteristic of this region probably have somewhat different causes, if
these two years are typical. The June period had a few heavy showers last-
ing some hours but many days with only light showers. The autumn period
had relatively many more heavy showers but the average intensity was
no greater than in the June period (cf. the discussion of rainfall intensi-
ties, supra). Although the June maximum is nearly as rainy as the au-
tumn one, the stability aloft is still marked in June whereas it is practi-
cally absent in the autumn. However, the lowest 1000-meter layer is very
unstable in June. It appears therefore that the spring rains are due to
heating of the surface by the increasing solar altitude which forces the con-
vection to penetrate the trade inversion occasionally with resulting intense
showers. The mid-summer rains are apparently of similar character, for
the solar altitude is a maximum again in July and instability in the surface
layers is high through August. But the autumn rains require less energy at
the surface since there is less stability aloft and any small convergence or
heating will easily bring a good shower. We note that the second rainfall
maximum is not in August, just after the second zenith-sun period, but
much later when cooling into winter has already progressed for several
months. This is not characteristic of most of the tropics nor of all of the
Caribbean region, and therefore calls for some comment. The cooling of
the North American continent in autumn is more rapid than the cooling of
the Caribbean Sea, so that relatively cool air masses begin to encroach on the
Caribbean from the north. There is as a result a tendency for convergence
over the Caribbean. The slightest disturbance of this sort to the air column
when it is very moist and in conditional equilibrium to high levels is apt to
set off ample rains.* This happens more and more frequently towards
winter and leads to greater and greater rainfall totals, until ultimately
the return of the trade inversion in December chokes off the convection in
the upper levels. The suddenness of the decrease in rainfall from Novem-
ber to December is characteristic and is obviously the direct consequence
of the sudden return of a strong trade inversion in December.
At times in autumn a large flat low pressure area hovers over the Carib-
bean for weeks, giving rise to almost steady rains over a wide region. The
gentle incessant convergence up to perhaps 6 or 10 km under such condi-
tions soon destroys any weak stability aloft that may have extended out
from the now retracted Azores High. Some years, however, have rela-
tively dry autumns when these conditions are presumably reversed, i.e.,
warmer than usual in North America and the High shifted more south-
westward than usual.
Examples analyzed in Externbrink, H., Kaltlufteinbruche in die Tropen. Archiv d. Deutschen
Seewarte, 57, nr. 7. 1937; and Culnan, R. N., in, "Curso para el Estudio de los Huracanes". U. S.
Dept. Commerce, Wea. Bur. Tomo 1: 85-90. mimeo. Wash. 1942.




66 SCIENTIFIC SURVEY OF PORTO RICO

Finally, we have to explain the large rainfall in February for the years
1937-38, which does not appear in the longer-period averages for this or
other Virgin Islands stations. This abnormal rainfall was associated with
an abnormal increase in the humidities aloft (FIGURE 9). The lapse rate in
February was very stable; however, in the course of February 1937 some
heavy rains occurred during a few brief periods when convection reached
to abnormal heights, which is the explanation for the great difference be-
tween the intensity per rainy day and that per rainy hour in that month.
The diagram of monthly mean relative humidities plotted against height
(FIGURE 9) is very helpful in the interpretation of the lapse rates, in par-
ticular for distinguishing the W layers that are stable (dry) from the un-
stable cloudy ones (moist). The rapid extension of the high surface hu-
midities into the upper levels between April and May is most striking; as
likewise the return to the winter dryness aloft between November and
December. The spring transition period, moreover, coincides with the
beginning of the seasons for some of the most severe instability phenom-
ena, such as hurricanes, high rainfall intensities, hail, and thunderstorms.
The question arises here whether the large accumulation of energy in-
dicated by the high summer temperatures and humidities of the surface air
parcels is sufficient to carry the low-level moisture to high levels by con-
vection, penetrating the upper stability, or whether the horizontal advec-
tion of air currents aloft from moister source regions (the Doldrums?)
does not labilize the upper layers enough to permit the surface air particles
to proceed to higher levels by free convection.* This problem cannot be
discussed until much more upper-air data from the whole subtropical re-
gion become available. It is interesting to note, however, the temporary
drop in relative humidity during June and July between 2500 and 5000
meters, an indication of the closer approach of the anticyclonic core.
During early winter the humidity reaches the remarkably high average
of over 90 per cent in the lowest cloud layers around 800 to 1000 meters,
owing to the steep lapse rates from the more rapid cooling into winter of
the lands (and the air over it) than of the sea, and to the "lid effect" of the
lower trade inversions ; in view of the error of the hair hygrograph there
must have been clouds or near-saturation almost every day in these layers.
It' is in March that the inversion is most intense, but by that time the fre-
quent cold-air invasions from the continent bring cooler and drier trade-
wind air (the wind roses on the Pilot Charts show fewer southerly com-
ponents in March than at any other time). Hence the low levels are rela-
tively driest in March.
See the interesting but speculative discussions by Externbrink, H., Met. Zeitsch. 54: 354-9,
413-17. 1937; and Scofield, E., Bull. Am. Met. Soc., 19: 225. 1938.





APPENDIX A


NOTES ON THE ORGANIZATION, INSTRUCTION, AND
HISTORY OF THE OFFICIAL D. W. I. RAINFALL
STATIONS, 1877-1917

According to records and correspondence now deposited in the National
Archives, Washington, the Danish West Indies Government early in 1877
acquired twelve rain gages with measuring glasses from the Danish Me-
teorological Institute, Copenhagen. Eleven observers in St. Thomas and
St. John were given gages and glasses, and a copy of the printed instruc-
tions of the Danish Meteorological Institute with amendments in ink indi-
qating how to construct the gage supports and to substitute the local units
of measurement, "lines," for the European metric ones. Blanks for enter-
ing and returning the recorded rainfalls were also sent out. The returns
were forwarded to St. Thomas and filed; a summary or copy was also
probably forwarded from St. Thomas to Copenhagen, but except for the
years 1877-88 in the "Set. Thomae Tidende," the reports seem never to
have been published.


DELIVERY OF OFFICIAL RAIN GAGES TO ST. THOMAS
AND ST. JOHN, 1877
Drafts of letter "No. 4", below, dated January 6, 1877, were sent in
English translation to Th. Stevenson, C. Danielson, Wesselhdft (Smiths
Bay), Rev. Warner, and Gottlieb, Esq., and in original Danish to Planter
Harthmann, Dr. Miller, and Capt. Leigh (all residents of St. Thomas).

According to your kind promise to take charge of a station for measuring the fall
of rain on your estate, I hereby beg leave to forward to you a printed copy of 'Direc-
tions for the measurement of the atmospheric precipitation (rainfall),' together with
12 monthly lists to be filled up in the course of the year, at the same time requesting
that you please send to the Barracks for a rain gage and appurtenances thereto and to
cause the same to be placed in a suitable locality on your premises. The lists, after
being filled up, are to be sent to Government Office at the end of each month.
Respectfully,
[unsigned]

The National Archives also contains a rough copy of the letter of Janu-
ary 6, 1877, "No. 4", to the Country Sheriff at St. John, forwarding 3 rain
gages, 3 copies of the printed instructions (see below), and a spare meas-





68 SCIENTIFIC SURVEY OF PORTO RICO

during glass (also monthly forms); one gage is for Cruz Bay, one for
Adrian, and one for Caroline (with consent of estate owners) ; monthly
completed rainfall reports to be sent to the Government, St. Thomas.
There is also a rough draft of a letter, dated February 7, 1877, to the
Country Sheriff at St. John, forwarding a reserve measuring glass and
requests that rain be measured at Cruz Bay with both the old and new
gage and reports sent in to St. Thomas.


INSTRUCTIONS TO D. W. I. RAINFALL OBSERVERS, 1877
Translated excerpts from "Veiledning til Moaling af Nedbor, Udor-
bejdet af Meteorologisk Institut (Copenhagen) 1873" (4 pp.), marked
original, D. W. I. copy of which is in the National Archives (translation by
Harold Larson).

The equipment consists of a supporting stand, a can, and a measuring glass.
The stand consists of four legs which are screwed fast at the top on each side of the
four-covered frame and lower down are secured to each of the arms of the wooden
crossbar [a drawing is entered on the margin of the copy in the National Archives].
The bottom ends of the legs of the stand are buried in the earth. It will be expedient
to fasten to each leg a small crosspiece, so that after being covered with earth the
framework cannot easily be pulled up or pressed down deeper into the ground.
The can is placed in the framework from the top. It consists of a collecting rim at the
top, therefrom a funnel and finally a container supplied with a spout (or lip) and a
handle.
The rim of the collector encircles a definite horizontal area (namely Ao square
meter). The original circular form of the aperture must therefore be carefully pre-
served. Its position must be horizontal and its height above the surface of the earth
6 feet, whereby the grounding of the framework must be directed.
The funnel, in addition to a vent, is also supplied with two (2) other small holes,
whereby air in the container can escape. These three (3) holes must always be kept
clean.
The tightness of the container should be examined into now and then by filling it
with water.
The spout should always be closed with a stopper in order to prevent the evapora-
tion of the rain collected or the flowing in of rain in this manner.
Measurement: by the depth of the rainfall is understood the depth to which the
fallen rain would stand if it remained lying as an even layer over a level surface of the
earth. This depth is given in lines, and the measuring glass into which the can is care-
fully emptied, after being removed from the framework is precisely so divided that
a quantity of rain caught by the collecting rim, and which makes a depth of rain of 1
line will fill the glass from one figure to the next. The space between the figures is
also divided by small marks into 8 parts, so that one can read thereon both lines and
eighths. Thus if the rainwater stands at 7 small marks above the figure 4 of the glass,
it is read 47/ lines. (While being read, the measuring glass must be held precisely
vertical.)
[NOTE: 1 mmn= 0.3115 of an English line; the Paris line used in Denmark was
never used in the Danish West Indies. There are exactly eight English lines to an
inch, but approximately 11.3 Paris lines to the English inch. Why the generally obso-
lete English line was so widely adopted in the D.W.I, is not known, but possibly it





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 69

was introduced by the planters from British countries or by merchants selling old
English measuring devices. R. G. S.]
[The supporting stands for the rain gages, according to marginal sketches and notes
on the Archives copy of these "Instructions", were constructed of pitch-pine sticks in
a tripodal design, to hold the upper rim of the gage at 36 inches above the ground (the
present standard height in many countries). Each stand cost $2.25 for the wood and
40 cents for painting, i.e., $2.65 per stand or $31.80 for the twelve gages.- R. G. S.]

LOVENLUND RAIN GAGE RECALLED, 1884
A rough copy of a request, dated October 12, 1884, to Planter C. Miller,
L6venlund, that the latter return to the Government Secretary the rain
gage issued to him at the close of the year, is on file in the National
Archives.

TRANSFER OF RAIN GAGE FROM EMMAUS TO EDEN, 1892
St. John, Country Sheriff's Office.
9 Mar., 1892.
This is to report the removal of the rain gage, hitherto set up in the yard of the
Pastor (Jacobs) at Emmaus; in view of Pastor's leaving the rain gauge was today
moved to Eden (a parcel of Emmaus), the owner of which, Quarter Officer (Rev. E.)
George, has promised to note rainfall and make monthly reports.







APPENDIX B

LIST OF HURRICANES PASSING OVER OR NEAR THE
VIRGIN ISLANDS*


YEAR ST. THOMAS ST. CROIx -YEAR ST. THOMAS ST. CROIX

1713 (?) (?) 1871 August 21 August 21
1738 August 30 (?) August 30 1876 September 13 September 13
1742 September 28 .... 1881 August 23 ....
1772 August 31-Sept. I August 31-Sept. 1 1889 .... August 16
1773 July .... 1889 September 3 ....
1775 .... July30 1891 .... August 18
1780 October 13-14 October 13-14 1893 .... August 16
1785 .... July 25 (?)1894 October 13
1793 August 12 1899 .... August 7-8
1804 September 3 September 3 1906 September 2 September 2 (?)
1809 .... September 2 1908 September 10 (?)
1819 September 21 .. 1910 .... September 6
1825 .... July 26 1916 July 14 July 14
1827 August 17 August 17 1916 August 22
1827 August 28 August 28 1916 October 9 October 9
1830 August 12 .... 1917 September 21 September 21
1835 August 12 August 12 1919 September 3 September 3
1837 .... July 27 1921 September 9-10 September 9-10
1837 July 31 .... 1924 August 17 August 17
1837 August 1-2 August 1-2 1924 August 28-29 August 28
1837 August 15 August 15 1928 .... September 12-13
1848 August 29-30 August 29-30 1931 September 10 ....
1851 August 18 .... 1932 September 26 ....
1852 September 24 September 24 1933 July 26
1859 .... September 2 1938 August 8 August 8
1867 October 29 October 29 1940 August 5 August 5

Compiled by R. G. Stone from the works of Garriott, Reid, Poey, Redfield, Alexander, Fassig,
Mitchell, and histories of the Virgin Islands. Since 1900 the U. S. Mon. Wca. Rev. gives complete re-
ports of storms. In later years (1900-) this list includes weaker disturbances which caused some rain,
wind, or swell.

TALLY BY MONTHS, 1738-1941

June July August September October November Total

St. Thomas 0 4 18 13 4 0 39
St. Croix 0 5 15 10 3 0 33
Both islands 0 8 22 17 4 0 51
(no duplication)



ACCOUNTS OF SOME VIRGIN ISLANDS HURRICANES

The following are quoted for convenience; see parts on Porto Rican
meteorology and references in bibliography for further information.


HURRICANE OF AUGUST-SEPTEMBER 1772

During the last days of August and the first days of September a hurricane passed
over the West Indies causing frightful havoc among the Leeward Islands. The disas-
trous effects of this storm were felt nowhere more forcibly than at Santa Cruz where,
it is said, the sea rose 72 feet above its usual height, carrying every ship at the island





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 71

on shore, some as far as 100 yards inland. Large stones were brought down from the
mountains, and there was a terrific electrical display. Four hundred and sixty houses
were thrown down at Christianstadts, and all but three at Fredericstadt. The maga-
zines and stores were quite ruined. The total damage [in the W. I.] was estimated at
$5,000,000. The damage at St. Thomas was placed at $200,000. (Garriott, Bull. H,
U. S. Wea. Bur. 1900.)


HURRICANE OF AUGUST 31, 1772
Excerpt from Alexander Hamilton's letter to his father describing the
St. Croix hurricane of August 31, 1772.

St. Croix
September 6, 1772
Honored Sir:
I take up my pen, just to give you an imperfect account of one of the most dreadful
hurricanes that memory or any records whatever can trace, which happened here on
the 31st ultimo at night.
It began about dusk, at north, and raged very violently till ten o'clock- then en-
sued a sudden and unexpected interval, which lasted about an hour. Meanwhile the
wind was shifting round to the southwest point, from whence it returned with re-
doubled fury and continued till nearly three in the morning. Good God! What horror
and destruction it is impossible for me to describe-or you to form any idea of
it. It seemed as if a total dissolution of nature was taking place. The roaring of the
sea and wind fiery meteors flying about in the air the prodigious glare of almost
perpetual lightning-the crash of falling houses and the ear-piercing shrieks of
the distressed were sufficient to strike astonishment into Angels. A great part of the
buildings throughout the island are leveled to the grounds almost all the rest very
much shattered several persons killed and numbers utterly ruined whole families
wandering about the streets, unknowing where to find a place of shelter the sick
exposed to the keenness of water and air without a bed to lie upon or a dry cov-
ering to their bodies- and our harbors entirely bare. In a word, misery, in its most
hideous shapes, spread over the whole face of the country. A strong smell of gunpow-
der added somewhat to the terrors of the night; and it was observed that the rain was
exceedingly salt. Indeed the water is so brackish and full of sulphur that there is
hardly any drinking it. . .
(Alexander Hamilton)


HURRICANE OF 1830
This storm or hurricane was severe at the Island of St. Thomas, on the night be-
tween the 12th and 13th of August, 1830. (Reid 1838.)


HURRICANE OF JULY 26 TO 27, 1837
Copy of a manuscript report at Lloyd's, dated St. Croix (Reid 1838).

About midnight on Wednesday, the 26th of July, it came on to blow smartly from
the east-south-east, shifting by Thursday morning, the 27th of July, to south-east,
blowing a gale of wind until towards noon, when it began to moderate.


(Signed)


Andrew Lang.





SCIENTIFIC SURVEY OF PORTO RICO


STORM OF AUGUST 2 TO 3,1837, AT ST. THOMAS

A. Observations quoted by DeBooy:


Hour Pressure (mm) Wind Direction and Remarks

1 P.M. 757
2 755
3 754
3: 45 753.3 N pressure began to drop rapidly
4 752 N strong gusts
5 747.5 NE storm increasing
6 744.5 NE hurricane force
6:30 740 NW "
6:45 731 NW "
7:05 731 NW
7:15 726 NW
7:25 718 NW
7:30 715 NW
7:35 713 calm
7:45 712 calm
8:00 712 calm
8: 15 711.8 calm
8: 20 712 S hurricane again
8:30 721 SSE "
8:45 728.5 SE (since 8:35)
9:10 731 SE hurricane
9:15 736 SE
9 20 740 SE
9:30 744.5 SE
9: 50 746.5 SE
10: 10 749.3 SE
10:40 751 SE
11:15 751.5 SE
11:30 752 SE
12:00 753
1:00 754 ....
2:00 755
3: 00 756 ....





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 73


B. Observations of Mr. Hoskiaer (quoted by Dove 1841, and Reid
1838):


Barometric Pressure
(in "lines";
Hour I line = 2.16 mm) Wind

Aug. 1; 18h Om 337 lines
Aug. 2; 2h 10m 335 NW
3 20 334 N increasing
3 45 334 N increasing
4 45 332 N tempest
5 40 331.5 NE
5 45 330 NE
6 30 328 NW
6 35 325.5 NW
6 45 324 NW hurricane
7 0 324 NW
7 10 322 NW
7 22 318.5 NW
7 30 317 NW
7 35 316.5 Dead calm
8 to 316 '" calm center
8 20 316 "
8 20 316
8 23 320 SSE
8 33 321 SE
8 38 322 SE
8 45 323 SE
8 50 324 SE
9 0 326 SE
9 10 328 SE hurricane
9 25 329 SE
9 35 330 SE
9 50 331 SE
10 10 332 SE
10 35 333 SE
11 10 333.25 SE
11 30 333.5 SE
14 45 335 SE
20 0 336.5 SW
21 0 336.75 E




C. Extract from the log-book of the brig, "Water-Witch", W. Newby,
Commander, from Liverpool to St. Thomas (kept by the mate), made by
Mr. Gilbert Ker, consignee of the vessel (in Nautical Time) :


Hour Course Wind Remarks on board, Wednesday, Aug. 2, 1837

P.M.
2 N.W.b.W. N.E. P.M. Fresh breezes and clear; people em-
4 played variously; made the island of
6 St. Kitts; in lower and all lee stud-
8 N.W.I/W. ding-sails.
10 At 2, made the island of Saba.
12 At dusk, in all studding-sails, Saba
bearing N.N.E.; and Eustatia E.N.E.;
at 8, in flying jib and royals; midnight,
fresh breezes and cloudy; in top-gal-
A.M. lant-sails.
2 A.M. Do. weather.
4 At 7, made the island of St. John's, and
6 shortly after that of St. Thomas.
8 Noon, squally; double reefed the top-
10 sails, and stowed the jib; the town in
sight.





74 SCIENTIFIC SURVEY OF PORTO RICO

D. Extract of a letter from Captain Newby, of the British brig, "Water-
Witch", from Liverpool to St. Thomas, and which left Liverpool, July 19,
1837.

Arrived off St. Thomas on the 2nd of August; morning squally, and the Water-
Witch was off St. John's, and standing for St. Thomas's, the wind north and north-
north-west. Noon, shipping in the harbour visible; at 1 P.M. squalls violent; at 3 P.M.
we had beat up within half a mile of the forts, when we could proceed no further for
the violence of the squalls, and anchored in ten fathoms water; sent down top-gallant-
yards, &c; did not suspect a hurricane. At 5 P.M. squalls ceased and began a heavy gale
of wind, at that time off the land. At 7 P.M. a hurricane beyond all description dreadful;
the windlas capsized, and I could not slip my cables, ship driving until I was in twenty
fathoms water; a calm then succeeded for about ten minutes, and then, in the most
tremendous unearthly screech I ever heard, it recommended from the south and south-
west; I now considered it all over with us, for the wind was directly on shore, and the
sea rose and ran mountains high. The foretop-gallant-mast (though struck) and the
gig were carried up some feet in the air, and the vessel drove again into twelve fath-
oms. We were obliged to steer her all night, and keep her head to wind, for when she
got her bows to it she went down on her broadside. At 2 P.M. the gale abated some-
what, and the barometer rose an inch; at daylight, out of forty vessels, the Wlater-
Witch and one other were the only two not sunk, ashore, or capsized.

E. Extract from the log of H-. 1\. S. "Spey".
Sunday, August 6, 1837
A.M. Arrived at Tortola. Here the hurricane has destroyed the town and several
plantations. One brig from St. John's, with a great number of small craft, total wrecks.
P.M. at 2:30. Came to an anchor in St. Thomas's harbour and landed the mails. Here
the hurricane of the 2nd appeared to have concentrated all its power, force, and fury;
for the harbour and town were a scene that baffles all description. Thirty-six ships
and vessels totally wrecked all around the harbour, among which about a dozen had
sunk or capsized at their anchors; some rode it out by cutting away their masts, and
upwards of 100 seamen drowned; but what was very extraordinary, there was not one
English vessel in the port. The harbour is so chocked up with wreck and sunken ves-
sels, that it is difficult to pick out a berth for a ship to anchor. The destructive powers
of this hurricane will never be forgotten. Some houses were turned regularly bottom
up. One large well-built house was carried by the force of the wind from off its foun-
dation, and now stands upright in the middle of the street. The fort at the entrance of
the harbour is levelled with the foundation, and the 24-pounders thrown down: it looks
as if it had been battered to pieces by cannon-shot. In the midst of the hurricane shocks
of earthquake were felt; and to complete this awful visitation, a fire broke out in the
back stores of Messrs. Stubbs and Co. Heavy tiles were flying about from the tops of
the shaking and trembling houses, killing and wounding many persons. One fine
American ship, 500 tons, was driven on shore under the citadel, and in an hour nothing
could be seen of her bhut a few timbers. Several fine merchant ships and brigs are at
anchor, dismantled, with cargoes; and not a spar or rope for standing rigging to be
had in the island. No place hitherto has suffered so much from a hurricane in all the
West Indies as St. Thomas's. Thank God we escaped so well out of it.
(Signed) R. B. James

F. Extract of a letter from Lloyd's Correspondent, dated at Santa Cruz,

On Monday, 31st July, 1837, the weather was moderate; several ships sailed on
Tuesday, the 1st of August; in the evening the wind was north-east and the weather





STONE: METEOROLOGY OF THE 'IRGIN ISLANDS 75

moderate. On Wednesday the 2nd, the wind during the night had shifted to the north;
the weather looked squally, cloudy, and suspicious, and continued so during the fore-
noon ; the wind shifted gradually to the north-north-west.
At 1 P.M. the falling of the barometer, the appearance of the weather, and the in-
creasing wind, left us no doubt of the approaching storm, and it came on from the
north-west, between 3 and 4 p.s. The mercury continued falling, and the gale in-
creasing until half-past 6 P.M. when the wind became westerly. At 7 P.M. the mercury
began slowly to ascend, but yet the storm increased in violence. At 8 P.-. it was blow-
ing a hurricane from west-south-west to the south-west, coming in furious gusts until
10 P.m., when a certain decrease in their violence had taken place, which abatement
continued until Thursday morning, the 3rd of August, when it blew a fresh gale from
the south.
(Signed) Andrew Lang

HURRICANE OF AUGUST 1837, AT TORTOLA
The brig, Jane, of St. John's, N.B., was driven on shore during the gale on the 2nd
of August. (Tortola, August 6.)
At Tortola the hurricane commenced at 3 P.,., and increased in violence until 9
P.M., when it began to abate. (Reid 1838.)

HURRICANE OF SEPTEMBER 2, 1859
On September 2, a hurricane of rather mild form passed over St. Kitts, that is the
center passed right over the island. It wrecked a number of boats besides other dam-
age. It passed over St. Croix after leaving St. Kitts. (Garriott: 1900. op. cit.)

HURRICANE OF OCTOBER 29, 1867
On the 29th October, 1867, at 7h A.M., the weather presented no unusual appearance.
There had been several showers of rain during the previous night.
At 1 lh A.M., the weather became very threatening, the barometer falling slowly, and
the wind blew occasionally in hard puffs from the N.W. At noon the wind had veered
to the W.N.W., and began to blow furiously. At 7h A.M., the barometer indicated a
pressure of 29.76 inches and it fell slowly and gradually until noon when the reading
was 29.64 inches. After the reading at noon it fell very rapidly until lbh 30m P.Mr., when
the reading was 28.50 inches. It remained stationary at this point until 2h P.M. when
it began to rise rapidly, and at 6h P.M. had reached 29.65 inches.
The wind blew very hard, capsizing or dismasting most of the small vessels in the
outer harbor, until lh 30m P.-M. when a lull or calm occurred, lasting thirty minutes.
At the expiration of the calm the wind commenced to blow from the S.S.E., the oppo-
site direction from which it blew immediately before the calm, with greatly increased
fury and power, and rain fell in torrents.
Previous to the calm only the lighter vessels in the harbor had suffered, but as the
wind increased the larger vessels were either driven from their anchors and swamped,
or stove on the rocks and piers, or were sunk at their moorings. Very few rode out the
storm, but of those that did the majority depended upon their anchors. The fury of the
storm, after the lull, lasted only about one hour, but in that short time a fearful amount
of destruction was accomplished. Between sixty and seventy vessels were driven
ashore or sunk at St. Thomas and vicinity, and many others were entirely dismasted
or otherwise damaged.
Nearly all the small craft in the harbor were lost early in the storm, and there are
no means of ascertaining the number of vessels of that class or the number of lives lost
in them. On shore the destruction was nearly equal to that in the harbor. Many houses





76 SCIENTIFIC SURVEY OF PORTO RICO

were unroofed, many toppled from their foundations, and others scattered in a con-
fused mass of rubbish. Vegetation was literally scathed. A metallic diving bell weigh-
ing several tons was carried by the force of the wind a considerable distance. The Gas
Works were completely destroyed. The large gas holder was rent outward near the
top, in an aperture about eight inches in diameter.
It has been estimated that at and about St. Thomas the value of the property de-
stroyed was between four and five millions of dollars, and that at least five hundred
lives were lost. More than one hundred and twenty bodies were recovered and buried.
The thermometer was not observed during the progress of the storm, but it is affirmed
by Mr. T. H. Jahnecke, who furnished the following data from his barometrical ob-
servations on the 29th, that the temperature did not decrease more than 4' during the
storm.

ATMOSPHERIC PRESSURE AT ST. THOMAs 29TH OCTOBER, 1867

Hour Pressure Hour Pressure

in. in.
7 A.M. 29.76 1.30 P.M. 28.50
8 29.75 2 28.50
9 29.73 3 28.90
10 29.72 4 29.20
11 29.69 5 29.48
12 M. 29.64 6 29.65
1 P.M. 28.86


The following list [of 79 ships, not reproduced] exhibits a statement of the casual-
ties to vessels at and near St. Thomas during the storm. . T. H. Jahnecke estimates
the velocity of the wind at St. Thomas at 74 miles per hour. . At St. Croix the
wind was N.W., W., and S.W. . it is concluded that, at the time the storm passed
St. Thomas the diameter of the destructive portion was about thirty-four miles . .
the diameter of the central calm was about 7.5 miles. . (Eastman 1868: 8-9,
13-14.)

HURRICANE AT SAINT THOMAS, W. I., AUGUST 21, 1871
The Island of St. Thomas, W. I., again claims public notice, and sympathy. On
Monday the 21st instant, a terrific Hurricane burst over the Island, causing more suf-
fering and misery among the poorer class of the Inhabitants than was experienced in
the Tornado of 1867. Since Sunday evening the weather bore a threatening aspect, the
wind blowing in strong puffs from N.N.E. to N.E., which continued all night until
Monday morning, when the squalls increased in violence, and the Barometer indicated
29.90. By 9 A.M., the weather gradually became more threatening, Barometer steadily
falling. At 1 P.M. the Quicksilver was down to 29 (in.) and the gusts of wind increased
in fury, blowing from N.N.E.; the Hurricane had now complete charge, heavy squalls
continuing until 3.30 P.M., when the wind veered to the North, and came down over
the hills in fearful whirlwinds, shifting from N.N.E. to N.N.W. From this time to
5 o'clock the worst part of the Hurricane was experienced, and nearly all the damage
to the eastern part of the town was effected then. The first part of the Hurricane was
now over, and a calm of about 30 minutes succeeded, as if the elements were prepar-
ing for a greater outbreak. At 5 P.M., the Barometer had reached its lowest point
about 28.10 to 28, the wind now changed to S.W. to S. and blew heavily, but the Mer-
cury commenced to rise gradually and the Hurricane passed away to the Westward.
The extent of damage which has been done in the Island has been very great; the
eastern part of the town has suffered most, and presents a desolate spectacle as the





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 77

Hurricane seems to have expended its utmost fury in that quarter, in some streets
only a couple of houses are standing, which are badly injured, whole squares of houses
have been blown down, and the ruins lie jumbled together -the Military Hospital
had the roof blown off ; the Roman Catholic Chapel in Cocoanut Square, is in a ruinous
state. The French Masonic Lodge, "Les Coeurs Sinceres," No. 141, was com-
pletely blown away, leaving only the walls of the foundation standing, but owing to
the activity of a large number of the Members, the greater part of the valuables of this
Lodge have since been saved from the debris, foremost amongst which, we may record
the library and Archives. The residences of the Dutch Consul, and of the acting
French Consul, have been unroofed, and otherwise badly injured. At Madam Bjerge,
only two or three houses are standing, this quarter, occupied by the poorer class,
suffered much. In fact the effects of this visitation are dreadful, in 1867, the damage
was more general, and not as concentrated as it has been in this instance -the lower
part of the town, the "Back-of-all," has suffered likewise, but not as much as the
eastern part already alluded to. In the harbour the loss has been trifling, in conse-
quence of the small number of vessels in port.
The American steamer "Florida," cut away both masts, to ride out the gale. The
French steamer "Sonora," was run ashore, after parting from her moorings. The
steam tug "Vice-Governor Berg" stranded at the "Haul-over," and a couple of other
small craft sustained some slight injury. The British barque "Duke of Wellington,"
with cargo sugars in hhds., lying off Prince Rupert's Rock, parted her cables, and
drifted towards Water Island, where the crew was safely landed. The vessel was
then blown about the Triangle rocks to the East of the entrance to the harbour, where
she was wrecked. The British barque "Jane Lamb" and Danish brig "Axelstad" col-
lided, sustaining some damage.
But what is most distressing, is the loss of life and personal injuries sustained by
many unfortunates, in the first blow; many of the poorer people living in the vicinity
of the parade ground and other streets were crushed by the falling houses, in one
house five persons were killed. By the Police Reports we learn that thirty bodies have
been discovered and buried, and that about seventy are maimed.
The Police Department acted with zeal and activity in rescuing the wounded, and
rendering assistance to the destitute; during Monday night and Tuesday morning the
Constables were indefatigable in their efforts to do good to the sufferers, and the
thanks of this Community is due to them, and the Military, for the promptness with
which the streets have been cleared, and broken houses removed, thereby precluding
theft, and permitting the necessary sanitary measures to be taken, as regards decom-
posing matter.
Our Colonial Council convened a meeting on the 22nd instant, and voted funds for
the relief of the sufferers, and many of those who lost everything, and might under
the circumstances have been without a morsel of food, have been at least placed above
want by the promptitude of our Representatives. The Mercantile Community, with
their accustomed liberality have nobly answered the appeal, and given freely,- the
place is seldom invoked in vain, as unfortunately the visitations of past years has too
well proved.
The distress and suffering have been very great, but it is hoped that the worst has
passed, and that we may be preserved from further calamities. Printed by A. Wal-
16e, St. Thomas, W. I. (Reprint from the "Set. Thomae Tidende", Aug. 23, 1871.)


STORM OF SEPTEMBER 12, 1876
Early in the afternoon of the 12th a telegram was received (at San Juan, Porto
Rico) from St. Thomas announcing that the barometer was falling rapidly there and
that, according to information from the island of St. Kitts, a fierce storm was raging
there, with signs of being a hurricane.





SCIENTIFIC SURVEY OF PORTO RICO


Data St. Thomas Vieques
Extent of barometric oscillation: 16 mm Hg 16 mm
Changes in wind direction: NE, E, NE NNE, NE, calm, SE, SSE
Hours of greatest intensity: 4 A.M., 13th 6 A.M., 13th


Thus the center of the storm passed S of St. Thomas, over St. Croix and Vieques.
It traveled about 35 km per hr. Three ships at St. Croix and seven at St. Thomas were
sunk or wrecked. (U. S. Mon. Wea. Rev. Suppl. 24, pp. 35-38.)

HURRICANE OF AUGUST 22 TO 26, 1881
The center of the storm was reported at St. Thomas at 3 A.M., August 23, and at
Turks Island, W. I., 3 c.M. on the 24th. (Garriott: 1900. op. cit.)

ST. THOMAS-HATTERAS HURRICANE OF SEPTEMBER
3 TO 12, 1889
From the reports of vessels sailing to the eastward of the Windward Islands, this
storm is thought to have been central September 1 in latitude north 14 and longitude
west 57. Passed over St. Christopher during the night of the 2d; barometer 29.50
wind northeast. The storm was central near St. Thomas on the 3d, and near to and
north of Puerto Rico on the 4th while on the 5th it had moved about two degrees in a
northwesterly curve away from Puerto Rico. (Garriott: 1900. op. cit.)

HURRICANE OF OCTOBER 7 TO 12, 1916
After a few days' respite another disturbance appeared in the vicinity of the Island
of Dominica, the barometer at 8 A.M. October 7 reading 29.84 inches with calm air
and rain falling. Notification was sent at once to the Windward Islands and to the
United States Naval Radio Service, and special observations called for. Nothing of
value was received during the day, but on the morning of the 8th it was apparent that
the disturbance was near and east of Puerto Rico and moving northwestward or
northward. Advices to this effect were sent to West Indian points and to Weather
Bureau stations on the Atlantic and Gulf coast and broadcast by United States Naval
Radio. No further reports were received until the morning of the 9th when the regu-
lar reports showed the storm to be still east of Puerto Rico. Special evening reports
afforded the first definite information as to the location of the storm center. These re-
ports were to the effect that the storm center had passed over the Danish West Indies,
Santa Cruz reporting pressure of 29.42 inches at 3 P.M. with a gale, and St. Thomas
29.26 inches at 6 P.M. This information was immediately given wide distribution over
the water and along the coasts and shipping warned to exercise great caution. By the
morning of the 10th the storm had recurved slightly and had passed to the northeast-
ward of Puerto Rico. Nothing further was heard from the storm until the captain of
the barque Bellas reported by mail that he had encountered it, in the form of a severe
hurricane with southeast to southwest gales, on October 12, in latitude 27 40' N.,
longitude 62 20' W. (U. S. Mon. Wea. Rev. 44: 583-584. October 1916.)

The "Set. Thomae Tidende" for October 14, 1916 remarked that the
toppling of the coal crane at St. Thomas gave the best indication of the
strength of the wind in the hurricane of October 9, since the crane was
built to withstand 180 km/hr.





STONE: METEOROLOGY OF THE VIRGIN ISLANDS 79

THE RAINFALLS RECORDED AT SOME STATIONS IN THIS STORM WERE AS FOLLOWS:
Barracks: Oct. 9 61 lines
Sekretariat: Oct. 9 30 lines
Oct. 10 25 lines
Cruz Bay: Oct. 10 60 lines, estimated, as gage was
blown down during storm.
Susannaberg: Oct. 10 59 lines

PRESSURES DURING THE VIRGIN ISLANDS HURRICANE OF OCTOBER 9, 1916*

Time Pressure Time Pressure Time Pressure

8:00 A.M. 29.80 4:40 28.99 6:10 28.70
12:00 m. 29.65 4:55 28.95 6:15 28.65
2:00 P.m. 29.50 5:00 28.90 6:40 28.73
2:50 29.45 5:15 28.75 6:50 28.70
3:00 29.35 5: 20 28.70 7: 10 28.80
3:15 29.30 5:25 28.75 7:15 28.85
3:30 29.15 5:45 28.70t 7:18 28.90
3:55 29.07 5:55 28.68 7:25 28.95
4:30 29.05 5: 57 28.65 7:30 29.00
Observations by Miss Emily Quin, Christiansted. St. Croix.
t A calm existed from 5:25 P.M. to 6:45 P.M. when the wind, which prior to 5:25 bad been blowing
from ENE, returned in great force from the SW.

STORM OF SEPTEMBER 3 TO 10, 1921

St. Thomas, September 11, 1921.
Dear Sir:-
From September 3rd to September 8th, 1921, light variable winds and calms, clear
weather, high visibility, St. Croix 40 miles away seemed quite near and distinct. Max.
temperatures 90 and 91 and very oppressive. Bright sunsets, red and copper colored,
pressure during these days about .05 below normal.
Sept. 8th. Small patches Cirrus and Cirrus-Stratus appeared in the sky from NE to
SE. Pressure slightly below normal with a downward tendency. At I P.M. received
first storm warning from Central Station at San Juan, Weather fine, light variable
winds.
Sept. 9th. At 8 A.M. the sea that had hitherto been very smooth, began to show
signs of a swell rolling in from SE. At noon sky began to get overcast, about 6.10 Ci.
and St. with patches of scud from E. At 6 P.M. swell increased considerably breaking
heavily on south coast. Wind from ENE to E about 18 miles per hour, sometimes
shifting to SE in gusts. Pressure 29.82, temp. 90.
Sept. 10th. At 3 A.M. tremendous swell from S breaking across mouth of harbor.
Brisk E to SE wind, velocity 20 miles. Between 10 A.M. and 2 P.M. wind and sea at its
height. Max. vel. 40 miles. At 6 P.M. signs of swell abating and coming from SW.
Overcast.
Sept. 11th. 6 A.M. Sea considerably smoother, wind SE. 20 miles. Cloudy. Pressure
29.84. At 4 P.M. marked improvement in weather generally.
Note: At no time during the passing of this storm was the barometer below 29.80.
Precipitation was practically nil.- Wm. 0. Simmons (Acting Special Observer,
U. S. Weather Bureau). (From letter to Dr. 0. L. Fassig, then in charge, U. S. W. B.,
San Juan.)

HURRICANES OF AUGUST 17 AND 28, 1924
During the latter half of the month two tropical disturbances reached the Lesser
Antilles from the region to the eastward. The first of these was centered between





SCIENTIFIC SURVEY OF PORTO RICO


Dominica and St. Lucia the morning of the 17th and the second a short distance
northeast of Dominica the evening of the 27th. The tracks of these two disturbances
were almost directly northwestward from the Lesser Antilles, the first at the rate of
approximately 270 miles and the other at 200 miles a day. The former continued to
move in a northwesterly direction until it reached a latitude 280 N and longitude
750 W. It then moved slowly in a westerly direction for 48 hours, after which it
turned abruptly and moved north-northeastward with rapidly increasing speed and its
course gradually changed toward the northeast.
The first tropical disturbance was of only slight intensity in the region of the Lesser
Antilles and of moderate intensity when its center passed between Puerto Rico and
the Virgin Islands during the evening of the 18th. It increased gradually, however,
both in intensity and size after passing to the north of Puerto Rico and within three
days, when its center was in about latitude 27, 36' N and longitude 74 30' W, the
winds near the center had increased to hurricane force.
The second tropical disturbance evidently developed much farther east than the first,
inasmuch as it was already a storm of considerable intensity when it appeared near
Dominica on the 27th. By the time it reached the Virgin Islands it had attained hurri-
cane intensity. The barometer fell to 29 inches at St. Thomas at 3 A.M. of the 29th and
great damage was done by the storm in these islands. A number of lives were lost,
hundreds of houses were destroyed and thousands damaged, and much damage was
done to crops. So great were the losses in the Virgin Islands that appeal was made to
the American Red Cross for substantial aid. (U. S. Mon. Wea. Rev. 52: 410-411.
August 1924.)
A tropical disturbance at the beginning of September was central about latitude
250 N and longitude 700 W. The history and subsequent movement of this disturbance
is discussed in the Monthly Weather Review for August [see above "Hurricane of
August 17, 1924"].
Later information which has just come to hand indicates that the center of this
storm passed between Antigua and Montserrat (Lesser Antilles) at 3:30 A.M. of the
28th. At 2 A.M. of the 29th the center with a reading of 28.56 inches passed over the
eastern end of the island of St. John. The western end of the island of Tortola ex-
perienced hurricane winds from 6 P.M. of the 28th to 6 A.M. of the 29th. The storm was
accompanied by torrential rains and by winds estimated about 100 to 110 miles an
hour. . serious damage resulted to crops along the path of the storm from Mont-
serrat to St. Thomas. The observer at St. Thomas estimated the wind at 110 miles
per hour from the north-northeast between midnight and 2 A.M. of the 29th. Esti-
mates of 100 to 110 miles per hour were also made at Montserrat and Antigua between
3 A.M. and 4 A.M. of the 28th. (U. S. Mon. Wea. Rev. 52: 464. Sept. 1924.)

Notes from the Daily Observation Book, San Juan, U. S. Weather Bu-
reau, August 27-29, 1924.
August 27, 1924:

Noon special observations were called for from all special meteorological stations,
except Santo Domingo and Puerto Plata. Special reports were also requested from
Antigua, St. Croix and Martinique. Request for vessel reports was made through the
Naval Radio Stations and several vessels reported giving their position, barometer
reading, wind direction and force, etc.
The following advisories were issued:
Advisory 2 P.M.- Moderate disturbance over the Virgin Islands at noon to-day,
apparently moving north west. Lowest barometer 29.82 inches at St. Thomas. The
center will probably pass north of Puerto Rico tonight or Tuesday morning. No high
winds expected over Puerto Rico.- Fassig.





STONE: METEOROLOGY OF T-IE VIRGIN ISLANDS


Advisory 6 P.M.- Unsettled weather continued to prevail in the vicinity of the
Virgin Islands. The disturbance is of moderate intensity and is moving very slowly
northwestward and apparently not increasing. No high winds are expected over the
Virgin Islands or Puerto Rico.- Fassig.
The above advisories were duly disseminated.
Cirrus clouds were observed moving from the southeast at 6 :30 p.t.
San Juan, August 27, 1924. Special observations were called for at 7 P.M. from St.
Croix and Antigua. Based on P.M. reports the following advisory was issued:
Advisory 8 P.M. Indications of a disturbance of moderate intensity apparently
centered near Dominica, Windward Islands. The disturbance will probably move
slowly westward with moderate to strong northerly winds and rain over the Virgin
Islands and Puerto Rico tonight and Thursday morning. Fassig.
The above advisory was duly disseminated.

August 28, 1924:
Noon specials were called for from all regular stations except Port of Spain, Santo
Domingo and Puerto Plata. Special observations were called for at 8 A.M. and noon
from St. Croix, Antigua, and Granada. Following hurricane warning issued for Vir-
gin Islands: Observer St. Thomas. Hoist hurricane warning 10 A.M. Storm central
over St. Kitts with barometer 29.56 inches. The storm will move slowly northwest-
ward over the Virgin Islands accompanied by moderate to high northeast winds and
rain today, changing to southerly winds to-night or Friday morning. Fassig.

Following advisories were issued:
Advisory 10 A.M. The storm increased in intensity during the night and moved
northwestward from Dominica to St. Kitts accompanied by moderate to high north-
east winds and rain throughout the Virgin and Windward Islands. The lowest ba-
rometer reported was 29.56 inches at St. Kitts. The storm will move slowly north-
westward over the Virgin Islands and Puerto Rico with moderate to high northeast
winds and rain today changing to southerly winds during tonight or Friday morning.
- Fassig.
Advisory at 3 P.M. At noon today the storm center was between St. Thomas and St.
Kitts and moving slowly northwestward. The lowest barometer reading reported
was 29.74 inches at St. Kitts. Moderate to high northeast winds and rain will occur
this afternoon or tonight over the Virgin Islands and over Porto Rico, especially the
eastern portion, changing to southerly Friday morning. Caution advised shipping in-
terests. Fassig.
The above warning and advisories were duly disseminated.

August 29, 1924:
Advisory 9 A.M. Storm center passed north side St. Thomas at 3:00 A.M. with
northeast gales shifting through north to southwest by 7 A.M. Lowest barometer 29.00
inches reported by U. S. Tug Grebe in St. Thomas Harbor. Storm center is apparently
moving slowly northwest. Moderate to strong southerly winds will prevail over the
Virgin Islands today and moderate northerly shifting to southerly over Puerto Rico.
Fassig.
The above advisory was duly disseminated.
(From Notes in Daily record U. S. Wea. Bur. San Juan.)











APPENDIX TABLE 1

REGISTER OF STATIONS AND AVAILABLE RECORDS


Station



Adrian Estate
Adventure Estate
America Hill Estate
Anguilla Estate
Annaly Estate
Anna's Hope Estate and Agric. Exper. Sta.
Anna's Retreat, on Charlotte Amalie
Estate
Barracks (Salut Batteriet), Charlotte
Amalie
00 Barren Spot Estate
Bethlehem (Old Works)
Bethlehem (New Works)
Betty's Hope Estate
Bonne Esperance Estate
Bonne Esperance Estate
Bourne Field, Lindbergh Bay (U.S.M.C.)

Calquhoun (hMt. Pleasant) Estate
Canaan Estate
Canaan Estate
Caroline(-berg) Estate
Castle Coakley Estate
Central Factory
Charlotte Amalie Estate (Anna's Retreat)
Charlotte Amalie (Town)t


Elev.
Island (feet)


Period of
observations known


Elements*


DANISH AND UNITED STATES VIRGIN ISLANDS


St. John
St. Croix
St. John
St. Croix
St. Croix
St. Croix
St. Thomas
St. Thomas
St. Croix
St. Croix
St. Croix
St. Croix
St. Thomas
St. Croix
St. Thomas

St. Croix
St. Thomas
St. Croix
St. John
St. Croix
St. Croix
St. Thomas


630
200
526
35
650
240
150-200?

10
140
95
200?
25
675
260
10

250
425
350
160
100
100?
200?


Fch. 1877-July 1900;1
Apr. 1935-March 19361
1911-: 1938-
1920-33
1911-; 1937-
1911-; 1920- (broken)
1911-; 1920-
1920-38
Feb. 1877-March 1917
1911-
1911-; 1937-
1911-; 1938-
1911-
1929-
1911-; 1922-36
Nov. 1935-

1911-; 1937-
1923-27
1911-
Feb. 1877-Aug. 1878
1938-
1938-
Feb. 1877-June 1880;
1938-


R; Rd
R; Rd
Rd
R; Rd
R; Rd, C, Wd
Rr, Rd, T, Wd,
E, C, H, P
Rd
Rd
R
R; Rd
R; Rd
R
Rd
R; Rd
Rr, Rd, T, Wd,
P, C, H, A, TI
R; Rd
Rd, T, Wd, C
R
Rd
Rd
Rd
Rd; Rd


References in
bibliography and to
locations of original datat



USA; 62
54, 62, ES, WB
62, 63, WB
54, 62, ES, WB
54, 62, 63, ES, WB
Wv, 54, 62, 63, 64, ES, WB, SCS
62, 63, WB
USA
54, ES
54, 62, ES, WB
54, 62, ES, WB
54, ES
62, 63, WB
54, 62, 63, ES, WB
Wv, USN

54, 62, ES, WB
62, WB
54, ES
USA
62, WB, ES
62, WB, ES
62, USA, WB


Key to Elements: R = rainfall (monthly totals only available); Rd = rainfall totals and number of rain days; Rr = rainfall intensities from recording gage; T = tem-
perature; Wd = wind direction; Wv = wind velocity; C = cloudiness; S = sunshine duration; P = pressure; 11 = humidity; E = evaporation; A = aerological observa-
tions; Th = thunderstorms.
t Key to locations of original data: USA = United State, Archives, Terr. Sect.; ES = Experiment Station, Anna's Hope, St. Croix; WB = U. S. Weather Bureau, San Juan,
or Wash., D. C.: USN = Bureau of Aeronautics, U. S. Navy Dept., Wash.; DMI = Danish Meteorological Institute, Copenhagen; TES = Experiment Station, Tortola,
British V. I.; SCS = Soil Conservation Service, San Juan.
$ See Barracks; Sekretariat; Custom House; Knox's Residence; Wall6e's Residence; Hornbeck's Residence; Kjaer's Hill.









APPENDIX TABLE 1 (continued)


Station

Christiansted Fort
Christiansted (Town, various residences)
Cinnamon Bay Estate
Cotton Grove Estate
Cowell's Hill (or Battery)
Cruz Bay (Christiansfort; Commis-
sioner's Residence)
Custom House, Charlotte Amalie
Dolby Hill Estate
Dorothea Estate
Eden Parcel (of Emmaus Mission)
Eliza's Retreat Estate
Emmaus Estate (Moravian Mission)
Estate Morning (Morning Star)
Fountain Estate
c Fredensborg Estate
Frederiksted Fort

Glynn Estate
Golden Grove Estate
Granard Estate
Great Pond Estate
Hornbeck's Residence, Charlotte Amalie
Jealousy Estate
Jolly Hill Estate
King's Hill, Police Station; Estate

Kjaer's Hill (Blackbeard's Castle), Char-
lotte Amalie
Knox's Residence, Charlotte Amalie
La Grange Estate
La (Little?) Princesse Estate
Liliendal Estate
Lindbergh Bay (see Bourne Field;
Mosquito Bay)
Louisenhoj Estate
Lbvenlund Estate

Lower Love Estate
Ma Folie Estate


Island


St. Croix
St. Croix

St. John
St. Croix
St. Thomas
St. John

St. Thomas
St. Croix
St. Thomas
St. John
St. Croix
St. John
St. Croix
St. Croix
St. Croix
St. Croix

St. Croix
St. Croix
St. Croix
St. Croix
St. Thomas
St. Croix
St. Croix
St. Croix

St. Thomas
St. Thomas
St. Croix
St. Croix
St. Thomas
St. Thomas

St. Thomas
St. Thomas
St. Croix
St. Thomas


Elev. Period of
(feet) observations known


25
20-80?

20
100
260
10-30?

27
200
775
40
160
50
125
360
200
60
80
90
65
20
20-100?
165
200
210

295

20-100?
30
20
1020
15

778
200

92
875


1938-
1852-1937 (broken)

Sept. 1878-April 1885
1920-23 (broken)
Feb. 1877-Jan. 1905
Jan. 1871-1917; 1917-
ca. 1890(?)-; 1917-
1937-
1939-
March 1892-Aug. 1917
1838-51
May 1885-Feb. 1892
ca. 1830-60?
1911-
1911-
1825-30; 1852-79, 1882-
1914; 1938-
1911-
1911-
1937-
1937?
1829-36
1911-
1911-; 1920- (broken)
1852-78; 1882-1914;
1911-; 1920-broken)
Feb. 1877-Sept. 1878
1845-47
1923-25
1933-34
Feb. 1877-Sept. 1883

1845-47?
Feb. 1877-Dec. 1879;
May 1905-Dec. 1909
1911-
Feb. 1877-Dec. 1882


Elements*


Rd, T, (Wd, C?)
Rd, T, Wd, P, C, H, Th

Rd
Rd
Rd
Rd; Rd, T, C, Wd

Rd, T, P, Wd, C, II, Wv
Rd
Rd
Rd
R
Rd
R
R
R
T; Rd, T, H, C,
Th, Wd; Rd
R
R
Rd
Rd
R, T, P
R
R; Rd, Rr
Rd, T; Rd; R: Rd
Rd

R, T, (Wd, P?)
Rd
Rd
Rd


References in
bibliography and to
locations of original datat


62, WB
9, 55, 62, 63, 65, 40, 34, 27, 22,
DMI, ES, WB
USA
62, 63, WB, ES
USA
65, 27, 62, 63, USA, WB, DMI

62, 63, 47, (27, 64?) USA, WB
62, WB, ES
62, WB
USA
34, 27, 65
USA
46
54, ES
54, ES
40, 27, 64, 55, 34, 22, 62, 63, DMI,
WB, ES
54, ES
54, ES
62, WB, ES
WB, ES
20, 34, 65, 25, 47, 27, 48
54, ES
54, 62, 63, 9, ES, WB, SCS
55, 65, 27, 40, 22, 54, 62, 63, 9,
DMI, ES, WB
USA

25, 34, 48, 27, 65
62, ES, WB
62, WB, ES
USA

25, 34
USA

54, ES
USA


















APPENDIX TABLE 1 (continued)


Station

Manning Bay Estate
Morning Star Estate (see Estate Morning)
Mosquito Bay (now Lindbergh Bay)
Mt. Pleasant (see Calquhoun)
Nisky (Moravian Mission)
Pearl Estate
Prosperity Estate, near Frederiksted
River Estate
o St. Georges Estate
-" Sekretariat (Administration Bldg.),
Charlotte Amalie
Sion Farm (near Anna's Hope)
Slob Estate
Southgate Estate
Strawberry Estate
Susannaberg Estate
Two Friends Estate
Wallbie's Residence, Charlotte Amalie
Whim Estate
Wintherg Estate


Experiment Station (Botanic Sta.)
(near Roadtown)
St. Bernard's Estate
Spanishtown (Rev. Murdoch's Res.)


Island


St. Croix

St. Thomas

St. Thomas
St. Thomas
St. Croix
St. Croix
St. Croix
St. Thomas

St. Croix
St. Croix
St. Croix
St. Croix
St. John
St. Croix
St. Thomas
St. Croix
St. Thomas


Tortola

Tortola
Virgin Gorda


Elev. Period of
(feet) observations known


so
10?

50
700-1000?
35
180
150
165

200
140
50
150
620
475
20-100?
90
700


1911-

1924-26

Feb. 1877-June 1889
Feb. 1877-Sept. 1882
1911-; 1923-37 (broken)
1911-
1911-
Jan, 1910-March 1917

1938-
1911-
1937?
1911-
Aug. 1900-Aug. 1917
1911-
ca. 1876?-95?
1937-
July 1938-


BRITISH VIRGIN ISLANDS
20 1901-

860 1831-33
50? 1909-20?


Elements*


R

Rd
Rd
Rd
R; Rd
R
R
Rd

Rd
R
Rd
R
Rd
R
R, T, P (?)
Rd
Rd, T, Wd, C


Rd, T

R, T
R


References in
bibliography and to
locations of original data


54, ES

62, 63, WB

USA
USA
54, 62, ES, WB
54, ES
54, ES
USA

62, ES, WB
54, ES
WB, ES
54, ES
USA
54, ES
47, 65, 27, WB
62, WB, ES
62, WB


21, 62, 63, WB, TES
48
21, (TES?)
















































1 I ','* 1 D. I 4. 1. n. 1 il: 6.6l 2.5 S.3 4l 5 4. T4 4 t3.12 1.-':

"It .9 39M .E,.. 5n.

J7.43 ,1549 2594
S 1 L.L ,1 io
4 J.1 .
41.4 2.3.r (l.;l
.- S.01.1 2s.7 -


: .. r ,,, ,, 0 l .1j L 47.9 65M.S 6. SdJiS 4..d (C..94)


TfUllwT*. r, 5D .
....=......... ..................................
"J I.'1 ill llqrkl."i gr. upi. mm" ", nr "i i.n. -i? T.i"i-in b' ir. -.rll .. i, ii-. .. ,-
dIH irr li-( h++ ofe lrjniti nr OII br4 ,-i lir .1 -:.r.2r,.,'; +ia:n -~i v .., .... K,.. "* -







APPENDIX TABLE 3
AVERAGE MONTHLY RAINFALL, ST. CROIX STATIONS, DANISH PERIOD 1838-1917, AND ST. BERNARD'S, TORTOLA, 1831-1833

Stations Jan. Feb. 1\ar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual

Eliza's Retreat' 2.02 1.74 0.87 2.29 3.41 2.53 2.48 3.54 4.09 4.42 4.74 2.57 33.63
Christiansted2 2.32 2.05 1.24 2.97 4.39 4.53 3.46 4.26 5.59 5.88 5.89 3.87 46.43
Frederiksted (Fort)3
1875 Rainfall 1.89 0.65 1.70 2.25 3.30 5.36 2.06 4.45 3.81 2.91 -
1875 Days with rain 20 7 13 15 18 15 17 20 20 18 -
1876 Rainfall 5.34 1.54 1.07 2.26 3.03 4.00 2.11 2.88 7.56 2.14 5.64 2.07 39.64
1876 Days with rain 26 16 12 8 20 11 15 13 15 16 18 17 187
1877 Rainfall 2.85 3.74 2.97 4.40 1.48 6.05 7.35 7.72 6.11 5.53 4.77 4.26 57.24
1877 Days with rain 26 9 11 10 8 19 20 19 19 21 20 20 282
1879 Rainfall 1.70 3.89 2.39 5.01 9.42 10.36 3.03 11.09 4.38 8.22 8.37 5.20 72.87
1879 Days with rain 18 12 9 19 16 14 13 13 13 16 24 14 181
Kings Hill'
1879 Rainfall 0.84 0.98 2.29 4.25 4.57 1.97 2.64 3.84 1.43 -
1879 Days with rain 7 9 11 19 13 17 20 17 15 -
Average for St. Croix* 2.38 1.91 1.70 2.60 4.47 4.01 3.65 4.70 6.07 6.56 5.23 3.53 46.89
St, Bernard's5 2.05 1.96 2.75 3.06 5.09 6.62 7.95 10.70 10.35 7.27 4.06 3.87 65.73
Co
'a Christiansted," 1852-1907 (in mm)
Average 59 51 40 69 122 116 97 120 154 176 142 96 1242
Greatest 147 183 170 193 473 321 395 356 442 466 468 329 1942
Least 10 2 7 4 14 9 13 28 41 51 11 27 665
Frederiksted,j 1852-1907 (in mm)
Average 62 45 48 69 111 101 93 127 154 173 130 84 1197
Greatest 148 112 175 303 471 318 248 277 553 545 300 202 1741
Least 9 3 3 7 17 9 9 38 22 45 37 25 728
Kings Hill,6 1852-Oct. 1878; Oct. 1888-1907 (in mm)
Average 59 44 40 62 103 92 92 109 149 156 125 87 1118
Greatest 137 128 173 247 446 244 264 292 486 409 370 284 1619
Least 5 4 2 5 16 10 17 24 38 52 21 25 682

St. Croix: Average of Christiansted, Frederiksted, and Kings Hill (same records as above)-table showing percentage of years that each month had the highest and lowest
rainfall of the year
Highest 1 0 1 1 13 8 4 10 21 26 12 3 100
Lowest 11 20 32 19 6 6 2 1 1 0 2 0 100

1 Average of 14-year period, 1838 -51. Observations of Major Long, quoted by Orsted; originally published in part in the "St. Croix Avis" and in the "St. Croix Agricul-
tural Reporter". Lowest annual rainfall was 16.75 in 1842; highest annual rainfall was 54.00 in 1850.
Average of 37 years- 1876, 1877, 1882-1916. See TEXT TABLE I.
3 From "IMeteorologisk Aarbog" (Copenhagen) pt. 2, for years 1875-79.
Christiansted, Kings Hill and Frederiksted, 1852-1911. See TEXT TABLE 5 for further details.
5 Average of 3-year period, 1831-33. From Schomburgk, 1837.
These are averages computed by Willaume-Jantzen from records he found in the Danish archives and Meteorological Institute; he converted them from lines to milli-
meters. We have not converted them into inches owing to the errors that would accumulate-with repeated conversions from casting off decimal remainders.







APPENDIX TABLE 4
AVERAGE MONTHLY RAINFALL TOTALS AND DAYS WITH RAIN, ST. THOMAS STATIONS
A. RAINFALL AT BARRACKS (SALUT BATTERIET), CHARLOTTE AMALIE

Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual

1877 5.23 2.38 4.75 0.81 5.80 10.53 3.62 6.94 4.73 11.75 3.83 (60.37)
1878 2.97 4.94 3.94 3.78 6.58 4.61 9.56 6.90 9.99 4.81 7.18 2.95 68.21
1879 3.25 2.66 3.69 6.90 11.16 19.15 4.53 4.12 3.70 9.19 10.70 1.87 80.92
1880 1.89 0.81 0.56 1.97 11.50 2.09 2.00 0.62 2.08 5.19 2.80 1.09 32.66
1881 2.58 0.37 0.31 1.28 8.78 6.47 1.06 7.25 8.09 7.90 6.19 3.03 53.31
1882 1.69 2.50 3.06 1.00 1.22 1.65 5.06 3.22 4.63 3.70 3.41 2.58 33.72
1883 3.94 2.95 4.39 4.36 9.33 1.50 3.23 9.28 7.63 4.89 7.00 6.78 65.28
1884 2.36 2.22 1.16 0.36 2.22 3.06 5.78 6.08 6.22 14.42 3.75 2.89 50.52
1885 0.81 1.52 0.39 4.05 1.47 1.87 1.91 8.64 2.14 5.12 8.89 5.22 42.03
1886 1.14 2.77 1.19 7.50 2.20 6.69 4.84 5.67 6.22 10.17 13.78 3.78 65.95
1887 2.64 1.53 1.05 4.29 4.05 5.42 6.20 5.03 3.86 1.76 5.32 0.40 41.55
1888 1.39 2.28 1.29 2.98 3.58 3.45 3.71 7.98 5.94 -
1889 1.39 1.56 4.58 11.81 9,19 1.91 3.89 10.78 3.23 3.14 1.67 (53.15)
1890 5.22 2.45 1.48 4.75 0.77 1.31 1.92 3.20 3.95 2.03 2.77 3.94 33.79
1891 1.47 1.61 0.56 1.50 1.91 7.02 5.25 8.61 2.45 11.23 6.11 2.39 50.11
1892 3.37 1.09 0.65 1.56 4.11 0.46 1.99 3.88 3.45 3.42 7.61 1.55 33.14
1893 0.65 2.66 1.20 2.83 3.50 5.02 4.06 9.22 3.71 3.41 [1.601 1.92 [39.78]
1894 2.06 1.34 1.37 2.15 4.89 2.52 7.37 1.79 5.28 10.83 3.53 4.61 47.74
^ 1895 1.76 1.02 1.39 2.02 4.75 1.87 2.60 4.73 7.78 9.94 6.80 13.63 58.29
0 1896 -
1897 5.34 0.31 0,94 3.98 14.69 1.68 2.36 1.90 6.03 2.72 3.48 2.39 45.82
1898 1.69 0.66 0.56 0.98 5.72 3.09 7.05 4.45 9.97 5.92 2.78 2.47 45.35
1899 2.86 0.83 1.09 0.89 1.67 2.59 2.25 14.56 1.94 5.92 8.42 0.72 43.75
1900 4.36 0.75 0,80 4.92 2.47 14.42 3.75 3.31 5.50 8.47 2.22 2.86 53.83
1901 3.55 1.12 2.88 0.59 4.88 7.16 8.48 2.19 4.61 7.34 7.97 3.66 54,42
1902 5.42 0.00 1.56 2.50 8.08 8.86 1.66 0.72 3.78 2.67 3.84 4.72 43.84
1903 1.12 1.34 0.89 3.89 1.48 4.36 2.69 3.50 5.62 4.28 5.75 4.91 39.85
1904 1.64 1.67 3.81 1.66 2.08 0.38 2.09 3.47 5.70 5.03 2.74 0.98 31.25
1905 3.62 0.41 2.25 1.31 8.08 0.91 3.80 6.06 3.61 5.38 4.78 1.28 41.49
1906 0.80 1.31 7.14 0.64 6.06 2.77 4.61 2.69 4.97 4.14 1.91 6.95 43.99
1907 1.38 4.97 0.72 1.72 1.02 2.09 3.14 2.64 2.08 5.98 4.34 5.31 35.39
1908 4.91 2.52 2.67 0.98 5.53 3.27 3.19 2.12 4.56 5.31 5.56 5.33 45.96
1909 2.38 4.67 3.61 1.78 1.17 4.67 1.38 9.92 3.62 8.16 11.55 3.69 56.60
1910 3.85 1.62 2.08 1.27 0.86 0.27 4.70 4.39 13.06 3.55 5.09 6.44 47.16
1911 4.16 4.11 0.38 2.38 6.39 0.61 3.09 1.25 1.86 5.74 6.56 14.50 51.02
1912 2.24 1.77 1.44 4.42 4.52 1.84 1.50 0.81 7.72 7.78 2.25 (36.32)
1913 2.08 3.19 3.45 3.25 4.17 2.50 1.25 1.50 3.77 4.91 2.50 2.84 35.41
1914 0.50 3.64 1.89 4.53 10.05 5.03 1.17 3.24 0.52 6.78 9.02 2.02 48.38
1915 1.84 2.45 1.74 6,77 5.25 4.80 1.62 2.31 2.58 2.42 8.81 1.86 42.46
1916 3.14 2.24 1.19 1.66 3.66 2.72 5.08 6.38 7.42 20.41 11.25 0.74 65.86
1917 1.56 0.34 0.59 -.-

Total 97.60 81.27 73.33 112.77 187.95 165.83 148.72 181.83 196.85 238.82 228.75 140.01 -
No. Years 38 40 40 39 38 39 39 39 39 38 38 38 -
Means 2.57 2.03 1.83 2.88 4.95 4.25 3.81 4.66 5.05 6.28 6.02 3.68 48.01
Note: ( ) [ ] see footnote to APPENDIX TABLE 2.










Year Jan. Feb. Mar.

1877 8 12
1878 14 8 11
1879 13 11 9
1880 18 5 6
1881 9 3 4
1882 19 11 14
1883 18 17 10
1884 14 11 10
1885 11 6 9
1886 5 14 9
1887 14 13 7
1888 9 7 8
1889 10 8
1890 17 17 12
1891 13 9 2
1892 13 5 9
1893 5 14 5
1894 14 10 13
1895 10 7 9
co 1896 -
N. 1897 10 5 7
1898 12 6 6
1899 16 10 7
1900 17 8 6
1901 11 9 12
1902 12 0 11
1903 13 10 7
1904 14 14 15
1905 11 7 14
1906 12 13 13
1907 13 12 10
1908 18 19 13
1909 13 17 12
1910 19 17 11
1911 21 10 3
1912 16 7 15
1913 21 13 19
1914 5 16 8
1915 12 11 7
1916 21 11 8
1917 10 4 9


Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual


405 380 357 461 485 532 506 542 571 616 521 -
40 40 39 38 39 39 39 39 38 38 38 -
10.1 9.5 9.2 12.1 12.4 13.6 13.0 13.9 15.0 16.2 13.7 152.2


Note: ( ) [ ] see footnote to APPENDIX TABLE 2.


Total 513
No. Years 38
Means 13.5










APPENDIX TABLE 4 (continued)

C. RAINFALL AT CHARLOTTE AMALIE ESTATE


Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.


1877 5.55 2.22 2.92 0.91 5.17 8.66 3.30 5.55 2.61 12.33 3.95
1878 3.17 4.94 2.25 4.83 6.16 4.98 6.36 4.84 9.90 5.03 6.22 1.96
1879 3.50 1.97 2.33 4.28 10.59 14.18 6.97 6.78 8.69 3.90
1830 0.25 2.31 17.37 3.25 -

Total 6.67 12.71 6.80 14.34 35.03 27.58 15.02 8.14 22.42 14.42 27.24 9.81
No. Years 2 4 3 4 4 4 2 2 3 3 3 3
Means 3.34 3.18 2.27 3.58 8.76 6.90 7.51 4.07 7.47 4.81 9.08 3.27


D. DAYS WITH RAIN AT CHARLOTTE AMALIE ESTATE


Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

1877 8 6 9 3 16 17 10 14 16 22 13
1878 12 9 8 7 9 11 15 9 18 13 13 8
1879 12 8 6 10 18 10 24 16 16 13
1880 3 7 19 13 -


Total 24 28 20
No. Years 2 4 3
Means 12 7 7


Note: ( ) [ ] see footnote to APPENDIX TABLE 2.


49 50 32 19 56 45 51
4 4 2 2 3 3 3
12 12 16 10 19 15 17


Annual

(53.17)
60.64





64.24


Annual


(134)
132












APPENDIX TABLE 4 (continued)

E. RAINFALL AT COWELL'S HILL (OR BATTERY)


Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual

1877 4.50 1.39 4.03 1.50 4.20 12.34 3.19 5.12 4.34 12.88 3.12 (56.61)
1878 2.33 3.75 2.45 3.86 5.80 4.12 6.48 5.34 9.69 3.73 5.64 1.72 54.91
1879 3.88 1.42 3.69 3.81 10.17 14.25 3.48 5.34 2.73 9.28 11.52 1.99 71.56
1880 1.65 0.89 0.51 1.20 8.67 2.95 1.86 1.29 1.79 6.19 2.44 2.83 32.27
1881 2.58 1.61 0.64 0.62 7.57 7.08 1.44 7.30 7.12 5.33 4.64 2.99 48.92
1882 1.69 3.15 2.03 0.53 0.75 1.28 5.00 2.43 3.55 5.80 4.58 1.79 32.58
1883 3.41 3.08 2.95 2.44 7.81 1.25 3.45 8.51 5.70 5.78 7.00 6.78 58.16
1884 2.43 1.83 1.08 0.02 2.77 2.92 4.58 4.67 6.31 12.40 2.55 2.71 44.27
1885 0.80 0.36 0.34 3.78 1.28 1.67 2.62 9.64 1.08 6.76 1.40 5.39 35.12
1886 1.62 3.56 1.94 7.02 1.39 3.67 4.26 4.53 5.42 9.42 11.86 3.30 57.99
1887 2.28 1.19 0.43 4.15 3.53 5.59 5.67 4.31 3.12 3.56 4.22 0.88 38.93
1888 1.39 1.61 1.31 2.09 3.23 4.06 3.89 7.98 4.65 -
1889 1.28 1.29 2.98 8.62 9.16 1.50 2.95 10.27 2.88 2.22 1.83 (44.98)
00 1890 6.08 1.91 1.52 3.70 0.67 1.09 1.20 2.80 1.81 1.76 1.92 3.19 27.65
1891 1.14 3.35 1.20 1.13 1.65 8.48 5.53 6.72 1.79 8.55 4.64 2.09 46.27
1892 4,14 1.28 0.55 1.42 4.37 0.34 2.17 4.98 2.78 4.03 7.14 1.29 34.49
1893 1.16 3.58 1.24 3.03 3.70 4.84 4.36 7.50 3.81 4.08 (1.63) 2.41 41.34
1894 1.09 1.14 1.26 2.70 3.78 2.43 6.94 2.56 3.45 10.70 2.23 3.89 42.17
1895 1.97 1.24 1.91 0.70 4.36 1.08 2.52 4.20 7.69 7.74 6.33 12.07 51.81
1896 .- -. -
1897 3.80 0.09 1.22 3.78 14.33 1.66 2.36 1.34 5.98 2.72 3.33 2.39 43.00
1898 3.52 0.31 1.11 0.81 3.61 2.80 6.23 3.28 8.87 4.30 2.22 1.94 39.00
1899 1.52 0.56 1.30 1.34 1.06 3.52 1.77 9.50 8.91 4.84 10.20 1.52 46.04
1900 2.64 0.50 0.94 3.48 1.77 6.70 3.30 3.20 4.03 5.20 2.30 2.60 36.64
1901 3.39 0.80 2.28 0.98 3.38 6.18 12.22 2.42 3.59 7.36 7.30 3.27 53.17
1902 5.50 0.00 1.12 2.80 5.75 6.34 1.53 0.75 2.06 3.00 3.55 4.31 36.72
1903 0.72 0.92 0.77 2.77 1.64 3.30 2.31 2.02 4.69 3.00 4.36 3.92 30.41
1904 0.88 1.77 3.70 1.28 1.16 0.53 1.16 2.66 3.75 5.34 3.06 1.17 26.46
1905 2.62 -

Total 64.23 45.68 40.17 66.44 114.31 111.47 110.17 121.41 129.76 148.09 131.14 81.38 -
No. Vears 26 27 27 27 27 27 27 27 27 26 26 26 -
Mleans 2.47 1.69 1.49 2.46 4.23 4.13 4.08 4.50 4.81 5.70 5.04 3.13 43.73

Note: ( ) [ ] see footnote to APPENDIr TABLE 2.













APPENDIX TABLE 4 (continued)

F. DAYS WITH RAIN AT COWELL'S HILL (OR BATTERY)


Year Jan. Feb. Mar. Apr.

1877 4 11 12
1878 9 5 7 12
1879 12 5 11 1
1880 15 7 5 4
1881 12 9 5 8
1882 16 9 10 3
1883 18 19 8 5
1884 17 15 9 1
1885 10 6 9 9
1886 8 14 8 6
1887 15 16 9 7
1888 7 5 5 9
1889 5 4 10
, 1890 19 12 13 15
0 1891 12 19 2 4
1892 12 5 8 10
1893 6 13 3 12
1894 10 14 13 10
1895 14 9 9 5
1896 -
1897 12 3 5 9
1898 16 4 8 8
1899 17 7 9 7
1900 12 8 9 11
1901 13 9 11 4
1902 12 0 6 14
1903 11 10 5 9
1904 11 14 14 8
1905 13 -

Total 329 246 216 207
No. Years 26 27 27 27
Means 12.6 9.1 8.0 7.7

Note: ( ) [ ] see footnote to APPENDIX TABLE 2.


May June July Aug. Sept. Oct. Nov. Dec.


288 320 355 320 353 353 359 334
27 27 27 27 27 26 26 26
10.7 11.9 13.1 11.9 13.1 13.6 13.8 12.8


Annual

(134)
137
136
117
155
117
171
151
107
162
125
(137)
156
144
131
121
144
142

137
125
141
145
163
128
138
133










APPENDIX TABLE 4 (continued)

G. RAINFALL AT KJAER'S HILL (BLACKBEARD'S CASTLE), CHARLOTTE AMALIE


Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.


1877 5.48 3.00 4.75 1.14 5.27 10.52 3.06 6.81 4.17 12.05 3.66
1878 2.28 5.39 2.64 2.88 4.47 3.81 6.29 4.88 6.52 -

Total 2.28 10.87 5.64 7.63 5.61 9.08 16.81 7.94 13.33 4.17 12.05 3.66
No. Years 1 2 2 2 2 2 2 2 2 1 1 1
Means 2.28 5.44 2.82 3.82 2.80 4.54 8.40 3.97 6.66 4.17 12.05 3.66


H. DAYS WITH RAIN AT KJAER'S HILL (BLACKBEARD'S CASTLE), CHARLOTTE AMALIE


Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

1877 8 11 14 6 17 16 9 9 15 15 14
1878 11 8 11 10 10 16 19 11 20 -


Total 11 16 22
No. Years 1 2 2
Means 11 8 11

Note: () [ ] see footnote to APPENDIX I


15 15
1 1
15 15


Ann


ual


(59.91)


Annual

(134)




152











APPENDIX TABLE 4 (continued)

I. RAINFALL AT LILIENDAL ESTATE


Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual

1877 5.45 3.17 3.70 0.89 7.69 13.75 3.34 7.52 5.34 12.22 3.78 (66.85)
1878 4.14 5.78 2.86 2.56 4.22 5.63 8.44 7.80 7.65 8.69 9.40 1.50 68.67
1879 3.47 2.09 4.50 7.09 12,59 17.89 4.63 3.92 4.26 8.97 6.94 1.20 77.55
1880 1.31 0.06 1.72 1.70 10.37 3.44 2.47 2.62 4.31 6.14 3.79 2.80 40.73
1881 1.48 0.00 0.34 1.55 9.52 9.02 2.38 8.75 8.19 7.81 6.09 1.25 56.38
1882 0.56 2.58 1,53 1.13 0.81 1.62 3.50 5.03 4.25 3.62 6.50 3.53 34.66
1883 13.94 6.94 5.59 11.06 16.43 -

Total 24.90 22.90 19.71 17.73 49.46 45.29 35.17 31.46 52.61 40.57 44.94 14.06 -
No. Years 6 7 7 6 7 6 6 6 7 6 6 6 -
Means 4.15 3.27 2.82 2.96 7.07 7.55 5.86 5.24 7.52 6.76 7.49 2.34 63.03




J. DAYS WITH RAIN AT LILIENDAL ESTATE


Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual

1877 12 8 9 5 16 16 11 9 16 15 13 (130)
1878 16 9 6 7 10 13 11 11 13 12 14 2 124
1879 10 6 14 12 15 15 12 7 10 19 16 9 145
1880 12 1 4 3 17 10 8 7 5 15 16 11 109
1881 11 0 1 5 14 18 6 10 13 14 11 3 106
1882 4 7 4 2 1 2 5 7 10 6 11 11 70
1883 20 16 11 14 14 -

Total 73 51 48 38 76 74 58 53 74 82 83 49 -
No. Years 6 7 7 6 7 6 6 6 7 6 6 6 -
Means 12 7 7 6 11 12 10 9 11 14 14 8 121

Note: ( ) [ ] see footnote to APPENDIX TABLE 2.




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