WHERE DO THE RAINDROPS FALL
Author: Eulalie R. Rivera Elementary School
Environmental Education Team
Grade Level: 6-8
4. Water 1. Social Studies
6. Resources 2. Math
Through map interpretation skills, students will complete the study guide
correctly, and make conclusions about the importance of rainfall to places and
1. Obtain road/political maps for the Virgin Islands from VI Division of
Tourism or the Territorial Government so that each student has one to work
with, or at best, share with one other student.
2. Familiarize students with reading/interpretation of special maps such as the
attached rainfall map.
3. Do a sample place location and rainfall interpretation.
4. Distribute the rainfall study guide for this activity and have each student
complete questions 1 and 2. (20 minutes)
5. Based on information from DCCA FACT SHEET No.2, discuss with students the
rainfall patterns over the Islands; how little of the rainfall is retained
and why (evaporation and runoff); and how important rain is to places and
people. After this discussion, have students complete question 3.
6. Go over all study guide items with the class.
Climatology and Oceanography of the Virgin Islands, DCCA Environmental Fact
WHERE DO THE RAIN DROPS FALL Study Guide
1. Use a Territorial Map of the Virgin Islands to locate the following places on
the attached map that show average yearly rainfall for the Islands. Compare the
location of the place and the rainfall legend and record the average yearly
rainfall for each place.
PLACE AVERAGE YEARLY RAINFALL
a) Round Bay, St. John
b) Inland, just south of Maho Bay, St. John
c) Cruz Bay, St. John
d) Cabrita Point, St. Thomas
e) Red Hook, St. Thomas
f) Coral World, St. Thomas
g) Charlotte Amalie, St. Thomas
h) Truman Airport, St. Thomas
i) Buck Island
j) Christiansted, St. Croix
k) Frederiksted, St. Croix
1) Your school:
2. As you study the rainfall map carefully, can you figure out for each Island
which side receives least and most rain.
a) St. Croix: least rain:
b) St. John: least rain:
c) St. Thomas: least rain:
WHERE DO THE RAIN DROPS FALL (cont.)
3. For all the Islands, the average yearly rainfall is only about 44 inches.
This is not very high.
a) Of the rain that does fall, how does evaporation and runoff diminish
the value of each rainfall?
b) Make a list of some problems people have on the Islands because there
is not much rain water to use.
DCCA ENVIRONMENTAL FACT SHEET NO.2
CLIMATOLOGY AND OCEANOGRAPHY
OF THE VIRGIN ISLANDS
Precipitation and Evaporation
In the Virgin Islands, rainfall varies over the year by island and by areas
within a given island. On the average, St. Croix receives 40 inches per year,
St. John 47 inches, and St. Thomas 42 inches. Heaviest rainfall occurs on
the western end of all three islands. The wettest months are August to November;
the driest from January to April.
Compared to other places, rainfall in the V. I. is relatively low. Evaporation,
however, is high due to constant wind and intense sun. This produces very dry
conditions over most of the islands excepting some high mountain forests.
Dryness and water loss are heightened by steep slopes which promote runoff and
shallow rocky soil which holds little moisture.
Runoff is accelerated by development,which strips the soil of water absorbing
vegetation and replaces it with non-absorbing pavement and hard surfaces. Rapid
runoff not only robs plants and animals of much-needed water, but also can cause
erosion, flooding and water quality problems (due to the soil and organic
materials which are carried by the water to the sea).
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Table 1 .--Monthly and annual average rainfall,
U.S. Virgin Islands [Data from
National Weather Service]
Month Rainfall, Inches
".CIt,: r" 6.18
The position of St. Croix relative
to St. Thomas and St. John has
been shifted north and west to
1 0 1 2 3 4
LI 25"- 30"
I 30"- 35"
IIII 35" 40"
S 40" 45"
- 45" 50
NOTES: 1: Sources: Water Balance of a Dry
Island by M.J. Bowden (1968) and Climate Water
Balance and Climate Change in the Northwest
Virgin Islands by M.J. Bowden et al..(1970)
2. For details, see original maps in the source
references or reproduction of the information in the
publication Virgin Islands of the United States
Resource Conservatin and Development Project.
V.I. Government (1973).
Water quality can be analyzed in a variety of ways depending on what information
is required. Common quality indicators are temperatures, salinity, dissolved
oxygen, transparency and bacteria. Additional measurements often required are
biochemical oxygen demand (B. O. D.), chemical oxygen demand (C. O. D. ),
nitrogen, phosphorous, silicon. The effects of these constituents on water
quality are significant to the well-being of individual organisms, whole
communities and entire ecosystems. All of these water components may assume
pollution roles if changed from normal levels.
Shoreline waters of the Virgin Islands have temperatures of 25.5-280C between
December April and 27.0 29.0C between June October. Salinity (the amount
of salt in the water) generally averages 35.5 36.2 parts per thousand and
34.0 35.2 parts per thousand during the same periods. Almost all our waters
contain dissolved oxygen near saturation.
Turbidity - (the amount of suspended particles in the water) is generally very
low; .indeed, the Caribbean is renowned for the clarity of its waters. In most
bays, the sea bottom is easily visible.
Runoff from the land, dredging, sewage, oil pollution and boat traffic are the
main causes of water pollution. Fortunately, ocean currents, waves, and swells
usually do a good job of flushing most bays. These forces, however, are
considerably reduced by the time they reach the heads of deep embayments. As a
result, circulation may be very poor in the inner reaches of some of our larger
embayments. These conditions are important because pollutants introduced to
these calm areas will be very slowly dispersed.
Generally, currents around the islands are driven by the North Equatorial or
Canary Current which moves through the Caribbean from east to west and
eventually joins the Gulf Stream off the south coast of North America. Local
currents in individual bays vary considerably due to exposure, winds, tides,
shoreline, and bottom geometry.
There is one high tide and one low tide per day on the north side or Atlantic
side of St. Thomas and St. John. There is a second cycle of high and low tides
each day on the south or Caribbean side of St. Thomas, St. John, and all coasts
of St. Croix. The difference between high and low tide, however, is very small
in the Virgin Islands (generally 10- 11", with seasonal high tides at
a maximum of two ft.). As a result, the currents caused by tides are not very
Waves and Swell
Most waves originate in deep water and are driven by the northeast tradewinds
most of the year. This produces an easterly swell on the windward or eastern
ends of the islands. The leeward or western ends of the islands are affected by
northern swells in the winter, and southeastern ones in the summer.
Wave action affects water circulation and water quality. It also moves sand and
other sediment around. This is why the amount and location of beach sand varies
Storm Waves and Tidal Flooding
Tidal flooding, created by major hurricanes having an average frequency of once
in 33 years, raise water levels from five to twelve feet above normal. A six-
foot height will flood lower parts of Charlotte Amalie, Christiansted,
Frederiksted, Cruz Bay, and other low-lying areas for 800 feet landward from the
shoreline. Besides flooding, damage to piers, houses, and other waterfront
facilities, and erosion of shores by storm waves can be heavy. Moreover,
passing hurricanes may create a nimus tide of as much as 1 foot below mean low
water that can temporarily affect grounding of vessels in shallow water.
A different, but equally threatening kind of wave is called a tsunami. These
are large sea waves of extraordinary length which are associated with submarine
seismic disturbances. These waves seem to occur about once every 10 or 15 years
in the Caribbean area.
100 '^ -d-'" Kingrish
- .- C
* frnchcap O/ O
General current patterns on the island platforms. From Dammann, etal, 1969.
Hurricanes and Prevailing Winds
Hurricanes constitute a seasonal threat (July to October) of potentially
catastrophic proportions. Most of the time, however, the Virgin Islands
is blessed by much gentler and more predictable tradewinds. These blow with
great constancy from the east and east northeast. It was the constancy of
these tradewinds that brought 17th and 18th century sailing ships to the Virgin
Islands and that made it a major trading center between Europe, Africa, and
the New World. Thanks to its tradewinds, the Virgin Islands is one of the most
popular places in the world for recreational sailboats.
The birth of a hurricane from an easterly wave
0 100 200 300 miles
0 100 200 30 400 km
A well developed hurricane
Island Resources Foundation, The Virgin Islands Marine Environment, ( V. L
Coastal Zone Management Program Technical Supplement No.1,1976). (Editor: Marsha
Mc,Laughlin, Policy and Planning Unit, DCCA. Further info: Environmental