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Title: climate of Alachua County, Florida
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Title: climate of Alachua County, Florida
Physical Description: Book
Creator: Dohrenwend, Robert E.
Publisher: University of Florida Agricultural Experiment Station
Publication Date: 1978
Copyright Date: 1978
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Table of Contents
    Copyright
        Historic note
    Main
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        Page v
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Full Text





HISTORIC NOTE



The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
(EDIS)

site maintained by the Florida
Cooperative Extension Service.






Copyright 2005, Board of Trustees, University
of Florida








September 1978 Bulletin 796



The Climate of

Alachua County, Florida

R. E. Dohrenwend



















Agricultural Experiment Stations
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
F. A. Wood, Dean for Research













The Climate of Alachua County, Florida

R. E. Dohrenwend

Visiting Assistant Research Scientist
School, of Forest Resources and Conservation
IFAS
and
Assistant Professor
Department of Botany
University of Florida, Gainesville, Florida










This publication was promulgated at an annual cost of
$1440.55 or a cost of 36 cents per copy to provide a source of
basic climatic data for persons concerned with resource and
environmental problems.







Dr. Dohrenwend is currently at Ford Forestry Center, Michigan
Technological University, L'Anse, Michigan 49946,



























ABSTRACT
An inventory of sources of meteorological data for Alachua
County is presented. Climatic data for the county is presented, the
local climatic patterns are discussed, and these patterns are related
to the zonal and local behavior of the atmosphere. Important prop-
erties of the local climate are related to problems of pollution, and
health.


















TABLE OF CONTENTS
Page
INTRODUCTION ......................................... 1
DATA SOURCES ......................................... 1
LOCAL CLIMATIC PATTERNS ........................... 3
Precipitation ......................................... 3
Pan Evaporation ............................... .... ... 4
Solar Radiation ....................................... 5
Temperature ......................................... 5
Relative Humidity ................................... 8
Winds, Ventilation, and Stability ....................... 8
CAUSAL MECHANISMS FOR LOCAL CLIMATIC PATTERNS 13
Drought .................................. ......... 15
CURRENT PROBLEMS OF APPLIED CLIMATOLOGY ....... 19
Health ............................................ 19
Pollution .......................................... 20
Computer Modeling of Topoclimate ..................... 22
DISCUSSION ................................. ...... ... 22
ACKNOWLEDGEMENTS ................................. 24
LITERATURE CITED .................................... 24











The Climate of Alachua County, Florida
INTRODUCTION
The local climate of an area is the product of the interaction
of the zonal properties of the atmosphere with the surface fea-
tures and properties characteristic of that area. Regional topog-
raphy and land-water distributions play important roles in the
determination of the patterns of variation in the climatic ele-
ments of a particular locale. Although northcentral Florida is a
relatively homogenous region geographically, Alachua County
differs from the counties surrounding it in latitudinal location,
distance from the sea, and in the percentage composition and
distribution of types of surface features. It is these differences
which let us discuss the climate at Alachua County as a distinct
entity.
Climatology is the study of climate. It explains the normal
behavior of atmospheric phenomena by means of physical princi-
ples, often with emphasis on atmospheric behavior at specific
points on the earth's surface. According to Stringer (1972)
there are three basic approaches to climatology. The first is the
presentation of climatic data. The second is the explanation of
climatic data in terms of physical principles. The third is the
application of analyses of climatic data to human problems
whenever possible. All of these approaches to climatology depend
upon the availability and acquisition of climatic data. Any cli-
matological study is only as good as the excellence of its data, so
it is logical to begin this report with an appreciation of the
sources of climatic data for the county.

DATA SOURCES
One of the best sources of climatic data for the Gainesville
area is the CAA meteorological facility at the Gainesville Air-
port. Hourly wind, temperature, humidity, pressure, and general
observations are available from this station. These data may be
obtained as microfilm copies from the National Weather Records
Center at Asheville, North Carolina.
There are three United States Weather Bureau Climato-
logical stations within Alachua County. One of these is located
at the University and is discussed below. The other stations are
located at High Springs and Island Grove. The High Springs
station collects temperature, freeze, and precipitation data. The
station at Island Grove collects precipitation data only. These
data are reported monthly and annually in the National Oceanic

1








and Atmospheric Administration's (NOAA) serial publication,
Climatological Data. Another NOAA publication (J. T. Bradley,
1972), provides general insights into, the patterns of Florida
climate which apply to the county.
The Forestry Department of the State of Florida collects
precipitation data at five locations within the county: La Cross,
Micanopy, Hawthorne, Gainesville, and a fire tower on Highway
41 between Archer and Newberry. The station in Gainesville
also makes daily temperature, humidity, and the more specialized
fuel moisture observations. The local division of the Forestry
Department is responsible for a four-county region and main-
tains stations at Starke and at Hollister on Highway 20 which
make the same daily observations as at the Gainesville stations.
The Regional Utilities Board (RUB) maintains weather sen-
sors at their Kelley plant in Gainesville and at the Deerhaven
plant on Highway 441 north of Gainesville. The RUB plans to
instrument a 1,000-foot television mast near the Devil's Mill-
hopper with temperature and wind sensors, in cooperation with
the County Pollution District. Data obtained from this tower
will constitute an important contribution, to the understanding of
local climate within the county, and they will be quite valuable
for application to problems of air pollution.
One of the richest and most useful sources of local meteoro-
logical and climatic data is the University of Florida. There are
several campus organizations which collect data, either on a
routine basis or for special studies. One of the most important
of these is the Institute of Food and Agricultural Science's
(IFAS) Climatological Station at the campus agronomy farm.
This unit reports to the NOAA Weather Data Center and also
publishes its observations in the form of a monthly data sheet.
Air temperatures and precipitation data from the Beef Research
Unit, located some three miles to the north of the Gainesville
Municipal Airport, are also reported in this data sheet. The data
included from the agronomy farm are: daily values of maximum
and minimum temperatures, minimum relative humidities, pre-
cipitation, soil temperature at 10 cm depth, pan evaporation, and
daily wind run. The data sheets may be obtained from Dr.
Gordon Prine of IFAS (Agronomy Department, 1969-1975).
Another valuable IFAS climatic data source is the Climato-
logical Laboratory at the University's Horticulture Unit. The
laboratory, under the direction of Dr. J. Bartholic, specializes
in freeze protection. In addition, it collects data on a routine
basis and is the site of other, very sophisticated microclimato-
logical investigations.

2








The School of Forest Resources and Conservation has re-
cently established a climatic station at the Austin Cary Forest
(October, 1975). This station collects wind vector, solar and net
radiation, pressure, air temperature, relative humidity, evapora-
tion, soil temperature, soil moisture tension, and depth to water
table data on a routine basis. In addition, the station is to serve
to supplement observation programs in forest microclimatology
and hydrology. Information concerning the station and data
availability may be obtained from Professors W. L. Pritchard
or W. H. Smith.
Dr. S. Davis of the Department of Botany has unpublished
information on local concentrations and distributions of air-
borne algae. Dr. H. Wittig of the Medical Center has collected
data on allergen and antigen concentrations and distributions
(Wittig, 1971).
The Interdisciplinary Center for Aeronomy and Atmospheric
Sciences (ICAAS), the Department of Environmental Engi-
neering, the Solar Energy and Energy Conversion Laboratory,
the Center for Wetlands, and the School of Forest Resources and
Conservation all have published and unpublished data available
from special studies. None of these organizations are involved
in programs of routine climatic observations at the present time.
There are very few upper air data, if any, available for the
Alachua County region, and none have ever been collected on a
routine basis. The nearest radiosonde or rawinsonde stations
are located at Tampa and Jacksonville, and any calculations in-
volving upper air conditions for Alachua County must use data
from one or preferably both of these stations. There are indi-
cations that aircraft soundings were made in the region during
the Second World War', but I have been unable to locate any
records of these soundings.
\' ~ LOCAL CLIMATIC PATTERNS ( i i
Precipitation
More than half of the average annual rainfall occurs during
the months of June, July, August, and September (Fig. 1). The
greatest amount falls during August which averages 208 mm.
Average annual precipitation for Gainesville is 1370 mm. The
month with the lowest average rainfall is November with 44 mm.
Rainfall may be extremely variable from year to year, and de-
partures from the mean precipitation of as much as 40 percent
have occurred. Snow occurs lightly, irregularly, and infre-
1H. P. Gerrish, personal communication.

3








250-



200-

AVERAGE ANNUAL
PRECIPITATION:
15 0 1370mm

E
E
100



50



\0 .. .. .
J F M A M J J A S O N D

MONTHS

Figure 1.--Average monthly precipitation for Gainesville in milli-
meters (30 years record NOAA).

quently. It may not occur as often as once every ten years, and
when it does occur, it will not normally accumulate or remain
in place.

Pan Evaporation
Average annual pan evaporation for Gainesville for 22 years
of records is 1674 mm (Fig. 2). Monthly variation in average
values follows the pattern of monthly variation in solar radia-
tion described below. True evaporative loss is less than the re-
corded pan evaporation, and depending upon the state of the
weather and condition of the vegetation may vary from 80 per-
cent to 20 percent of the recorded value.2 This element varies
less from year to year than does precipitation.
2J. F. Bartholic, personal communication.

4








200
ANNUAL AVERAGE
1674 mm

150



E
E 100



50



S I I I I I I I I I I I I
J F M A M J J A S O N D
MONTHS
Figure 2. Average pan evaporation for Gainesville in millimeters (22
year record, 1954-1975, IFAS).

Solar Radiation
Daily totals for solar radiation were supplied by Dr. Erich
Farber of the Solar Energy Laboratory for the years 1955-1975.
These data were summarized on a monthly basis, and an average
value for solar radiation was obtained for each month. These
values were summed to provide an estimate of average annual
solar radiation. This procedure was necessary because of gaps
in the data. The average annual solar radiation for the Gaines-
ville area was 156,150 langleys (Fig. 3). May receives the
greatest monthly solar radiation, and January the least. The
greatest year to year variation occurs from June through Octo-
ber, with the strongest variation occurring in June.


Temperature
The average range in monthly temperature is about 130C,
and the average diurnal temperature variation is also about
130C (Fig. 4). The average diurnal range is somewhat reduced
during the summer months due to the depression of the average
maximum temperature by afternoon cooling caused by thunder-

5











18
17404


16307 15553 15619

15
S- 14999


S/13703 13305

12061





0
12-


'J / \10009
g 9945
E 9 8765
S8480



6
I I I I I I I ,l l i I
J F M A M J J A S O N D

MONTHS

3





0 -
Figure 3. Average monthly solar radiation for Gainesville in langleys
(20 year record, 1955-1975, Solar Rad. Lab.).


storm activity, and by the presence of freely evaporating sur-
faces (Priestly, 1966). In the absence of such cooling, maximum
shade temperatures may go as high as 400C. In the winter, a
temperature as low as -90C has 'been recorded. This is a rare
event, once in 30 years, and on the average a freeze may be ex-
pected only four times per year (Tatble 1). The average frost
free season for Gainesville is 295 days. There is an average of
1,108 heating degree days (calculated from a base temperature



6








"0C F


S *54 6, EXTREME
'43 o "'51 MAXIMUM
30-- 86
AVERAGE
i ,-, \ MAXIMUM
: 20-68 1 ..-- P -2
I-5 MONTHLY
S62 \ AVERAGES
O 10- 50 62
"AVERAGE
LU 0 '- '58 MINIMUM
I-
0 32 g 2

3 '56 EXTREME
"-10- 14 '67 '62 MINIMUM
I, I I I I I I I I I I I
J F M AM J J A S 0 N D

MONTHS
Figure 4. Average, maximum and minimum air temperature for
Gainesville in C (30 year record, 1939-1968, NOAA).
Extreme maximum and minimum temperatures shown with
year of occurrence. 210C Average Annual Temperature.


of 650F) during the winter months, which means low average
heating costs for the county and its residents (Fig. 5).
The average annual soil temperature at the 10 cm depth is
23C (Fig. 6), and average diurnal variation runs from 4C to
2.50C. Average annual variation is about 160C. The warmest
month is July and the coldest is February at that depth.


Table 1.-Freeze Data, (30 Year Record) Climate of Florida.

Threshold Mean Date of Mean Date of Mean Number Number of
Temperatures Last Spring First Fall of Days Occurrences
oC OF Occurrence Occurrence Between Dates Spring Fall


0 32 2/14 12/6 295 27 21
-2.2 28 1/30 12/16 320 21 18
-4.4 24 1/8 12/26 352 9 8
-6.7 20 4 2
-8.9 16 1 1



7











c, 300
u >-
0 200

Cr 100
a 0 I I I I I I ,
J F M A M J J A S O N D
MONTHS
Figure 5. -Average heating degree days for Gainesville, calculated
from a base temperature of 650F (18.50C) (15 year
record, NOAA).

30
30
AVERAGE ANNUAL
S----w---- -- ----_ ---SOIL TEMPERATURE
S20- \\
4 AVERAGE MAXIMUM
a: AVERAGE MINIMUM
a. 10 -
I--.
o I l l I I Il l l I
J F MA M JJ A S ON D
MONTHS
Figure 6.-Average monthly maximum and minimum soil tempera-
tures for Gainesville for 10 cm depth under short pensa-
cola bahia grass sod. (5 year record, 1969-1975, IFAS).


Relative Humidity
Average monthly minimum relative humidity rarely falls
below 40 percent. There are a few days each summer when rela-
tive humidity remains above 70 percent. When these values coin-
cide with high temperatures, both human and animal physi-
ologies are highly stressed. More work needs to be done on the
collection and presentation of relative humidity data for the
county. Because data are presented as daily minimum relative
humidities, calculations of average monthly relative humidities
were not possible.

Winds, Ventilation, and Stability

Based on data from the local airport from 1970 through 1972,
Alachua County is a region of light winds (Fig. 7). Ninety-five

8












JAN FEB MAR APR

I.I II ,II. 1. 50-
S 6 1 6 1 6 1 6 W
U

25 $

MAY JUN JUL AUG
I I I | i 0
1 6 1 6 6 6




SEP OCT NOV DEC
1 I. I I
I 6 1 6 I 6 I 6

24,6 1 1-3 Knots
57 2 4-6
3 7-9
4 10-12
5 13-15
6 16-18
7 19-21
Figure 7. Monthly wind speed distribution- Gainesville Airport (3
year record, 1970-1972, NOAA).


percent of all winds are less than 12 knots, 78 percent of all
winds are less than 9 knots, 56 percent of all winds are 6 knots
or less, 'and 22 percent of the time there is no measurable wind.
The average wind speed for the three years of records analyzed
was 3 knots, or 3.1 m/sec. Gusts of 40 to 50 knots, enough to
cause light structural damage, were observed during the period
analyzed.
There is a pronounced diurnal variation in wind speed (Fig.
8). During the winter season (October-April), nights are usu-
ally calm, with 90 percent of the winds occurring between 8 p.m.
and 8 a.m. less than 3 knots. Wind speeds ordinarily reach their
maximum values between noon and 4 p.m. During the summer
season (May-September), there is a shorter calm period. Be-
tween 10 p.m. and 7 a.m. over 90 percent of the winds are less
than 3 knots. Wind speeds may be expected to attain maximum
values between 1 p.m. and 5 p.m.
An analysis of the diurnal variation in wind direction shows
that the normal wind direction is from the north at night for

9











1001 Winte r








U),


2 50 V












4 8 N 16 20 M
I..
0

Summer
100







I
















4 8 N 16 20 M
HOURS
0

















4 a N 16 20 M
HOURS

Figure 8. Diurnal variations in wind speed Gainesville Airport (3
year record, 1970-1972, NOAA).

10











the entire year. During the day, the wind is almost equally likely
to occur from any direction. At night, during the summer sea-
son, the winds are least likely to come from the southwest, west,
or northwest. This description is based on only three consecutive
years of observations and should be regarded as indicative of
the patterns of variation which may be encountered. It is not a
definite description of these patterns.
Except for a slight tendency for more northerly winds
during the winter months, and more westerly flow during the
spring, the direction of wind flow appears equally likely from
any point of the compass (Fig. 9). Subjective observations indi-
cate that there may be a preferred direction of flow at certain
times of the day. Such patterns would not be revealed by the
monthly wind roses, and were not observed in the analysis of
wind direction data for 1970.
Ventilation heights, the height of the column of air in which
vertical mixing takes place, have been computed for Gainesville
and the Alachua County region from Jacksonville and Tampa
rawinsonde observations (Table 2). A maximum average height
of about 1,400 meters occurs in May, and a minimum average

"2 23 3? / so P2 27
JAN 4 \4 FEB OMAR 3 APR a33 2 I
2 / 40 9 24
30 29 33



20 24 3 260 34 3026 25
MAY 24 24 JUN 7 34 JUL 3 45 AUG 27 35
S32 42 32 36 3 26 30
34 38

21 25 25
"25 22 23 ,6
SEP 27 23 OCT 2 25 NOV 25 22 DEC 3 25



W E o 25 so
"% Occurrence

Figure 9. Monthly distributions of wind direction Gainesville Air-
port (3 year record, 1970-1972, NOAA). Numbers refer to
strongest recorded gusts (knots), zero line is for calms.

11









Table 2.-Air Quality Data -Ventilation Capability.

Mean Wind Mean Maximum Mean Seasonal Relative
Month Speed Mixing Depth Ventilation Ventilation Dilution
(m/sec) (meters) (m2/sec) (m2/sec) Factor

Dec 3.0 810 2430
Jan 3.3 730 2420 2390 1.00
Feb 3.5 950 3320


Mar 3.6 940 3380
SApr 3.4 1310 4450 4250 1.78
May 3.5 1410 4930


Jun 3.2 1360 4350
Jul 2.6 1310 3400 3790 1.59
Aug 2.8 1290 3610

Sep 2.6 1270 3300
Oct 2.9 1290 3740 3310 1.39
Nov 2.9 1000 2900








height of about 750 meters occurs in January. If we assume that
these heights are controlled by synoptic scale phenomena, the
extrapolation of Tampa or Jacksonville values probably gives a
fair approximation of the true ventilation heights for the
Alachua region. The weighted mean ventilation height for
Gainesville based on both Tampa and Jacksonville soundings is
about 1,350 meters.
Wind speeds used for the computation of ventilation in Table
2 are mean monthly wind speeds, not mean vector wind velocity.
The mean vector wind velocity, unlike mean wind speeds, takes
changes in the direction of motion into account. That is, it pro-
vides information concerning the true rate at which a unit
volume of air will move from its point of origin. Thus, mean
wind speeds for Alachua County will be larger than mean vector
wind velocities, and will overestimate the ventilation capability
of the atmosphere in this region.
The atmosphere of Alachua County displays a high degree
of stability. Environmental Science and Engineering, Inc. has
computed the annual Pasquill stability frequencies as part of a
pollution study for the RUB. Their figures indicate a high de-
gree of stability, with 71 percent frequency for stable and very
stable conditions.
Gainesville has low level inversion conditions (0-150 meters
height) during 40-50 percent of the hours per day in the fall.
This is an average for the period 1936-1960, and it corresponds
rather well to periods of occurrence of a low-level nocturnal
inversion. However, there were only 50 to 60 cases of severe
stagnation occurrences (four days or more of stagnating anti-
cyclone) for the same period (Pack, 1964). As discussed below,
other mechanisms also contribute to the maintenance of a stable
atmosphere above the county.

CAUSAL MECHANISMS FOR LOCAL CLIMATIC PATTERNS
There are two major sets of factors which act to determine
the climatic patterns of Alachua County. The first set consists of
the critical features of the surface of the county and their loca-
tion in relation to the geography of the region. The second set
consists of the predominant features of zonal atmospheric be-
havior.
In the first set, we may list: 1) latitude, 2) proximity to the
Gulf of Mexico and the Atlantic Ocean, 3) presence of inland
lakes, 4) strength of surface heating, which depends on a variety
of surface properties, and 5) the rate of nocturnal cooling.

13








In the second set, we may include: 1) sea breeze convergence,
2) frequency of frontal passages, 3) frequency and strength of
hurricanes, 4) frequency and duration of anti-cyclonic subsi-
dence conditions, 5) frequency and intensity of occurrence of
the trade wind inversion, and 6) the position and strength of the
North Atlantic Subtropical High. The behavior of this last fea-
ture of the atmospheric circulation controls the local behavior of
most of the remainder of the set.
Although it does not have an effect on the dynamics of tro-
pospheric behavior, there is one additional feature of the atmos-
phere at the latitude of Alachua County which is quite important.
The ozone layer here is not as thick as it is over the remainder
of the United States.3 As a result, the solar radiation in the lati-
tude of the county is richer in ultraviolet radiation than in other
parts of the United States.
The average year in Alachua County may be divided into two
seasons, 1) a warm rainy season and 2) a cooler dry season. The
warm rainy season runs from about the middle of May to the
end of September. The cooler dry season dominates the re-
mainder of the year. About 60 percent of the rain falls during
the hot summer months, occurring as afternoon thunderstorms
generated by strong surface heating, and fed by a double sea
breeze convergence. Florida is not wide, and as the Atlantic and
Gulf sea breezes approach each other from either side of the
peninsula, they force air upwards. When high cloud cover in-
hibits convective development in the afternoon, permitting only
the formation of small cumulus clouds, rain may occur at night
as a result of instability generated by nocturnal radiative cooling
from the tops of the small clouds. Precipitation during the sum-
mer has a very patchy horizontal distribution for any particular
day. It is not yet known whether some areas within the county
have greater average rainfall than others.
Frontal passages during the winter months are the most
variable rain producing mechanism for Alachua County. Frontal
or low occurrences within Florida average 38 for winter, 29 for
spring, 19 for summer, and 41 for fall, for the years 1965-67
(Shaw, 1968). Shaw's winter and fall are included in the cooler
dry, winter season defined above. During the winter months, the
differential seasonal cooling of land and sea, the occasional pres-
ence of stagnated high pressure cells, and the formation of low
level inversions by the high rate of nocturnal cooling act to
maintain a high degree of atmospheric stability. A high percent-
3A. E. S. Green, personal communication.

14








age occurrence of the trade wind inversion during these winter
months (70 percent in February) also contributes to this sta-
bility (Dougherty et. al., 1967). Under these conditions, convec-
tive activity is suppressed and possibilities for vertical mixing
are limited. Usually the entire county will receive rain as a re-
sult of a frontal passage. The rain may occur at any time of day
since frontal storms are not dependent upon local land surface
heating.
Following the movement of a cold front across northern
Florida, the lower troposphere will be dominated by colder air
with relatively warmer air (higher potential temperature) aloft.
Such a configuration is stable and acts as an additional inhibitor
of vertical mixing.

Drought
A decrease in the frequency of frontal movement across
northern Florida is one probable cause for periodic drought oc-
currence. A reduction in the frequency of frontal storms will re-
duce total annual precipitation substantially below annual values
of evaporative demand as estimated by pan evaporation. Rain
accompanying frontal storms is usually less intense than that
associated with convective activity and will tend to be more ef-
fective for the recharge of soil and surface storage. On the
other hand, the intensive rainfall associated with late afternoon
convective storms will tend more to recharge the limestone
aquifer, particularly in built-up areas where the water runs off
rapidly to enter the aquifer through solution sinks. A substantial
reduction in the number of frontal passages will cause extensive
surface drying with concommitant vegetation stress, lowering
of lake levels, and the depletion of shallow wells.
Figure 10 shows an extension of the Bermuda High across
the Florida peninsula to the western shores of the Gulf of Mex-
ico. Figure 11 shows the strong substance and atmospheric sta-
bility associated with this feature as displayed by a sounding
made at the Kennedy Space Center. The broad ridge of high
pressure associated with the extension has become a closed high
which will most probably move northeastwards into the southern
United States, or change into a high pressure ridge which may
then recede eastward into the Bermuda High. Although this
example is based on one occurrence for which data was available
to the author, the type of atmospheric structure and its effects
are typical of the presence of the Bermuda High at all seasons.
The Bermuda High and associated subsidence are strongest and

15










138




144


150
153 -










Figure 10. Synoptic situation for 850 mb 18 December 1974,
showing an extension of the Bermuda High across the
Gulf of Mexico. Contours are 10's of meters above sea
level.
present most often during the winter months, and the occur-
rence illustrated is typical of its presence at this season.
The ridge extension of the Bermuda High is also common
during the summer months (Sands, 1966), and ordinarily would
induce very arid conditions within the Florida peninsula. Were
it not for the intense surface heating and the presence of large
bodies of warm water on either side of the peninsula, and the
relative weakness of this feature during the summer months,
Florida would be as arid as the great sub-tropical deserts at the
same latitudes. The ocean and the Gulf provide moisture and the
differential land-sea heating provides a pressure gradient for
the development of sea-breeze convergence which powers intense
afternoon convective storms.
A strengthening of the ridge extension and the concomitant
increase in the associated stability (Fig. 11) would also act to
reduce the amount of annual rainfall. This is a second mechan-
ism which could act to cause drought conditions within the
county.

16









10,000


8000

DEWPOINT DRY BULB
6000 ,TEMPERATURE TEMPERATURE


z 4000
-

2000 ------- INVERSION BASE


0-
-30 -20 -10 0 10 20 30
TEMPERATURE (C)
Figure 11. Atmospheric structure associated with the synoptic situa-
tion on 18 December 1974.

Records (NOAA, 1898-1972) indicate that drought in Ala-
chua County and in much of north Florida results from summer
precipitation deficiencies. Major droughts occur on the average
of once every seven years (Table 3). A drought year is normally
characterized by an extended dry summer with higher tempera-
tures than usual. Usually, maximum temperatures in Alachua
County are limited by the energy used for evaporation from sur-
faces ordinarily well supplied with water (Priestly, 1966). As
surfaces in the county dry under drought conditions, maximum
temperatures rise to values well above normal. A year of major
drought is often proceeded or followed by one or two years of
drier than normal weather, intensifying the impact of the
drought.
It is interesting to note that summer drought conditions are
sometimes associated with an unusually cold winter (NOAA,
1898-1972). It has been suggested, based on floristic evidence,
that Florida was drier than at present during the Wisconsin
glaciation.4 This may have resulted from the larger surface area
of the peninsula during that period, or from regional atmos-
pheric circulation patterns similar to those associated with pres-
ent drought-cold occurrences.
4D. B. Ward, personal communication.

17









In addition to a possible strengthening of the subtropical
high pressure system, a high located to the west of Florida, as-
sociated with a trough along the east coast extending to the
Florida peninsula, could bring cold polar air south. This is simi-


Table 3.-Timing of Drought Occurrence.
Intervals Between Occurrences
Year Severity Years between Years between
each occurrence severe occurrences

1898 weak
1899 severe
6 1899-1906 6
1906 severe
1907 severe
1908 severe
5
1914 weak 1908-1917 8
1915 weak
1
1917 severe
1918 weak 1917-1923 5
4
1923 severe
1
1925 weak 1923-1927 3
1
1927 severe
3 1927-1931 3
1931 severe
1932 severe
1933 weak 1932-1943 10
9
1943 severe
1944 weak 1943-1954 10
9
1954 severe
1955 severe
5
1961 weak 1955-1963 7
1
1963 severe
2
1966 weak
4 1963-1971 12
1971 weak
4
1976 severe -
Average Interval 3.93 years 7.11 years


18








lar to the 'smog chute' discussed later. A mechanism of this
form, when coupled with increasing frequency of continental
polar air mass movement across the plains, might well act to
decrease the effect of frontal precipitation while maintaining
cold, clear air over the peninsula. Increased frequency of cold
air mass movement southwards during the winter, associated
with anticyclonic circulation aloft over the southeast during the
summer, would give the drought-cold associations observed. To
my knowledge, this association of circulation patterns has not
been demonstrated; it remains an hypothesis to be tested.
Summer rains have a very fortunate cooling effect in the
afternoons. Two mechanisms contribute to this effect. The after-
noon storm clouds act to screen the surface from solar radiation,
and there are also downdrafts of cold air from the storms. Al-
though most of the water which falls on the county during the
summer comes from the Atlantic Ocean or the Gulf of Mexico,
Alachua County, situated as it is, inland in the northern part of
the state, does not experience strong temperature modification
effects from either body of water.
Although Alachua County may be expected to experience
hurricane force winds only rarely (winds causing structural
damage have occurred between 10 and 15 times in 55 years)
(Bradley, 1972), hurricanes in the proximity of the Florida pen-
insula will usually produce rain in the county. Percent frequen-
cies of tornados and other intense local storm occurrences are
unknown.
As a result of nocturnal radiative cooling, ground fogs may
form in the late evenings during the winter months. These fogs
will usually last into the early morning, only rarely persisting
into-the afternoon. The top of the ground fog marks the bottom
of the temperature inversion produced by the nocturnal radia-
tion loss. Early morning temperature inversions are especially
frequent and may act as effective concentrating mechanisms for
local nighttime pollution emissions.

CURRENT PROBLEMS OF APPLIED CLIMATOLOGY
Health
Because of the increased ultraviolet radiation at the latitude
of Florida, the probability of skin cancer resulting from pro-
longed exposure to the sun is higher here than elsewhere in the
nation.
Alachua County has three periods of peak pollen activity,
which may be quite unpleasant for sufferers from hayfever and

19








similar respiratory ailments. These periods are: 1) January-
February, 2) April-May, and 3) July-September. Not only is the
third period the longest, but pollen concentrations are highest at
this time (Wittig, 1971).
There are usually several days every summer in. Alachua
County when high temperatures, high humidities, and intense
solar radiation create conditions which are severely stressful to
both humans and animals. Such conditions may be particularly
serious for the ill, aged, or pregnant. Any heavy outdoor labor
should be avoided at these times. Unfortunately, cooling degree
day data are not available.

Pollution

The Florida Department of Pollution Control (1974) has
estimated that by 1985 ambient particulate levels for Gainesville
will be in violation of the 24-hour concentrations of the state
ambient air standard and average annual concentrations of both
state and federal ambient air quality standards (Table 2). En-
vironmental Science and Engineering, Inc. have completed a
model simulation study of air quality in Alachua County for the
RUB. This study has been referenced earlier. The ICAAS Caper
project in 1971 measured concentrations for particulates S02,
NO2 and total oxidents in the Gainesville area.
The Regional Design Studio has mapped air pollution sources
for the county from aerial photographs and from an aerial re-
connaissance. The resulting map (not shown) shows that while
the most severe sources appear to be point sources, most of the
visual deterioration of air quality is due to internal combustion
emissions from area sources. The largest area source is provided
by traffic loads in the Gainesville area. From these area sources,
pollution concentrations will vary in strength during the day,
displaying peak values during hours of heavy traffic. Emission
and concentration patterns will change at these times, extending
out away from Gainesville along commuter routes.
Another mechanism postulated by Gerrish (1972) not only
acts to concentrate pollutants, but may also serve to bring pollu-
tants into the county from distant sources. Gerrish terms this
mechanism a 'smog chute.'
In a smog chute, the persistent and perhaps convergent flow
from the north establishes an inversion at middle levels of the
atmosphere over Florida. This inversion acts as a lid prohibiting
dispersion upwards [Fig. 12]. Such inversions aloft are common
with this type of flow pattern. The sinking and mixing of the air

20








below the inversion aloft would be confined to the peninsula by
the sea breezes blowing inland from both coasts. Thus the geogra-
phy of Florida helps to establish a unique atmospheric funnel for
carrying possible pollutants from north of Florida down into our
state.




SMOG
CHUTE













DEW POINT TEMPERATURE

_/ ,DRY BULB
TEMPERATURE

0 L-- -- SUBSIDENCE OR
\ TRADEWIND INVERSION



TEMPERATURE -








Figure 12.- Pollution concentration by convergence. The inset dia-
gram shows the vertical atmospheric structure associated
with this process.

21








Alachua County is located in the center of the state and could
be quite susceptible to such incidents. Fumes from pulp. mill
operations at Palatka or Perry may infrequently be detected by
the inhabitants of Gainesville. Although Palatka is 42 miles to
the east and Perry is 84 miles to the west, the fumes are still
evident and objectionable upon the rare occasions that they
reach Gainesville.
There is a general feeling within Florida that meteorological
conditions in the state are particularly favorable for the dis-
persion of air pollution and the maintenance of excellent air
quality. This idea has been given formal support in the 1972
State of Florida Air Implementation Plan. In spite of this, I am
forced to conclude that Alachua Couny owed its relatively clean
air to a paucity of emission sources, not to' any particularly fa-
vorable meteorological circumstances.

Computer Modeling of Topoclimate
Using the Dynaspace computer model of land use patterns of
Alachua County, a map of the spatial distribution of eight pat-
terns of temperature behavior was generated (Dohrenwend and
Wetterquist, 1977), One of the most striking features of the
resultant topoclimatic map was the prediction of a 'heat island'
around the city of Gainesville (Fig. 13). Ground measurements
confirmed the existence of this heat island, and its pattern seems
to, correspond closely to that predicted by the computer. The
strength of the heat island effect was about 30C.
The Dynaspace model has also been used to generate a first
order model of air pollution patterns within the county. Although
this model needs quantitative verification, the predicted patterns
have been confirmed by aerial observations. This model demon-
strates that the two major present sources are automobile emis-
sions and electric power generation. As it continues to be cali-
brated and improved, the Dynaspace model is becoming an in-
creasingly powerful tool for an examination of the effects of
changing land use patterns on local climate and air quality.

DISCUSSION
The material above is largely descriptive in nature, and is
presented in the hope that it will provide a useful summary of
the climatic behavior of Alachua County. Because of the multi-
plicity and obscurity of data sources, it is often very difficult to
acquire a useful picture of the overall climatic behavior in a
restricted area the size of a county. An understanding of local

22































Figure 13.- Computer predicted heat island for the Gainesville area.
Numbers on the diagram refer to temperature observa-
tions in 'C. The measured 4.50C isotherm shows close
agreement with the, predicted maximum temperature ef-
fect (shaded area).

climatic patterns is frequently crucial to the success of agricul-
tural and environmental studies, an important consideration
since the University of Florida is located at Gainesville. In addi-
tion, the data presented provide a base for practical applications
in the fields of air pollution control, agriculture, regional land
use planning, and public health.
In addition to the material presented, some studies of other
aspects of the local climate are currently in progress. The Re-
gional Design Studio is presently generating predictive maps of
topoclimate as responses to different land use planning strate-
gies. The Regional Design Studio is also working on a model to
generate predictive maps of regional air quality patterns. Dr.
J. Bartholic of the Fruit Crops Department is currently working
on different aspects of freeze protection. Dr. A. E. Green and
his Interdisciplinary Center for Aeronomy and Other Atmos-
pheric Sciences are continuously developing research involving
a variety of problems concerning local atmospheric conditions.

23








Work is being initiated on the chemical quality of precipitation
and dry atmospheric deposition.
Before we can describe the local climate of a region the size
of Alachua County in really adequate detail, much work remains
to be done. A detailed study of the coupling between local varia-
tions in climatic elements and changing synoptic patterns would
be a valuable contribution. Research on the spatial distributions
of the different climatic elements and their relationships to local
topography should be continued. For example, a study of rain-
fall distribution within the county would be quite valuable. Ul-
timately, Alachua County could act as a natural laboratory for
the development of ideas for the general prediction of local cli-
matic behavior.


ACKNOWLEDGEMENTS
The author wishes to acknowledge the support and encourage-
ment given by Dr. A. E. S. Green and his ICAAS group at the Uni-
versity of Florida. I also wish to acknowledge the support given by
Prof. O. F. Wetterqvist and his Regional Studio at the University.
Sidney Cowles, Neil Hall, and Amelia Ensenat provided invaluable
assistance with the compilation and reduction of much of the data
presented.


LITERATURE CITED
Agronomy Department. 1969-1973. Climatological Data for
Gainesville, Florida and B.R.U. University of Florida.
IFAS. Gainesville, FL.
Bradley, J. T. 1972. The climate of Florida, Climatography of
the United States No. 60-8. U.S. Department of Commerce
NOAA. Environmental Data Service. Silver Springs, MD.
___ 1971. Florida Climatology. In: Report for the Co-or-
dinated Program for the Remote Sensing of Atmospheric
Aerosols. ICAAS. University of Florida. Gainesville, FL.
pp. 4-16.
Dohrenwend, R. E., and 0. F. Wetterqvist. 1977. Prediction of an
Urban Heat Island from Land Use Patterns. Urban and
Regional Planning Series, No. 8. University of Florida,
Gainesville. 28 p.
Dougherty, H. T., L. P. Riggs, and W. B. Sweezy. 1967. Character-
istics of the Atlantic Trade Wind System Significant for
Radio Propagation. Institute for Environmental Research.
IER 29-ITSA 29. ESSA Technical Report. Boulder, CO.
p. 75.

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Farber, E. A. 1971. Solar Radiation Measurements at the Univer-
sity of Florida Solar Energy and Energy Conversion Lab-
oratory. In: Report for the Co-ordinated Program for the
Remote Sensing of Atmospheric Aerosols. ICAAS. pp.
64-76.

Florida Department of Pollution Control. 1974. Air Quality Main-
tenance Area Designation Report. Tallahassee, FL.
Gerrish, H. P. 1972. Air Pollution in Tropical Florida. Miami
Interaction. Vol. 4, No. 1. pp. 21-24.
NOAA. 1898-1972. Annual Summaries-Climatological Data-Flor-
ida. U.S. Department of Commerce. Environmental Data
Service. Silver Springs, MD.
Pack, D. H. 1964. Meteorology and Air Pollution. Science. Vol.
146, pp. 1119-1128.
Priestly, C. H. B. 1966. The Limitation of Temperature by Evapo-
ration in Hot Climates. Agr. Meteorol. 3 (1966) 241-246.
Sands, R. D. 1966. A Feature-of-Circulation Approach to Synoptic
Climatology Applied to Western United States. Technical
Publication 66-2, Department of Geography, University of
Denver, CO. 332 p.
Shaw, A. J. 1968. The Climatology of Air Pollution Potential.
Hillsborough County Health Department. Tampa, FL.
Stringer, E. T. 1972. Foundations of Climatology. W. H. Free-
man and Co., San Francisco. p. 586.
Wittig, H. J. 1971. Atmospheric Fungal Antigens. In: Report
for the Co-ordinated Program for the Remote Sensing of
Atmospheric Aerosols. ICAAS. pp. 42-47.




















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