Title: Climatological Aspects of Rainfall
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Title: Climatological Aspects of Rainfall
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Spatial Coverage: North America -- United States of America -- Florida
 Notes
Abstract: Richard Hamann's Collection - Climatological Aspects of Rainfall
General Note: Box 12, Folder 1 ( Materials and Reports on Florida's Water Resources - 1945 - 1957 ), Item 18
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Full Text







Climatological Aspects of Rainfall

by
WERNER A. BAUM* AND STANLEY E. ASPLUNDT


Introduction
The availability of fresh water must be a basic
consideration of any treatment of water-management *
problems. As long as it is not economically reasonable
to convert salt water to fresh water artificially, and as
long as artificial stimulation of precipitation remains
in experimental stages, we shall have to rely primarily
on the fresh water supplied by rainfall at times and
places and in quantities provided by nature.
The past must be the guide to the future, and it
is the responsibility of the meteorologist and clima-
tologist to maintain, evaluate, interpret and assist in
the application of our precipitation records.
The purpose of this paper is to provide some gen-
eral but necessarily limited background on the distri-
bution and reliability of rainfall in Florida. It must
be emphasized that a generalized background discus-
sion of this type is not intended for use in the solu-
tion of any specific water-management problem. It
appears to be a truism that the more general a climatic
study, the less its utility in the solution of specific prob-
lems. Optimum use of climatic information for oper-
ational purposes requires a study specifically tailored
to the special requirements of the operational purpose.
It is therefore to be hoped that engineering needs will
be met not by unwise use of information such as given
here, but by utilization of available professional advice
in the intelligent incorporation of the weather factor
in the solution to the particular problem.

Meteorological factors in the rainfall regime
The gross features of the rainfall regime of Florida
can most readily be interpreted in terms of the large-
scale air motions in the low and middle levels of the
atmosphere, the water-bearing layers extending from
the earth's surface to about 20,000 ft. above it. As a
prelude to detailed examination of observed Florida
rainfall, it is desirable to sketch the meteorological
characteristics of our location in those terms.
Winter. In the winter, Florida is near the southern
extremity and under the influence of a broad stream
of generally westerly currents of air which flow around
the hemisphere at latitudes from about 25 to 60 degrees
north. Frequent undulations of varying sizes in this

*Head, Department of Meteorology, Florida State University.
tDepartment of Meteorology, Florida State University.


stream impel an alternation of northward and south-
ward components in the westerly flow; and within
this mid-latitude belt there is frequent occurrence of
more or less rapid transitions between the warm air
masses of lower-latitude oceanic regions and cold air
whose particular properties depend upon its history
in terms of contact with continents and cold oceans.
The southeastern United States is frequently invaded
by cold air from Canada, and sometimes by cool air
from the northern Pacific. The transition zone be-
tween the advancing cold air and the warm air is
called a "cold front." Displacement of warm moist air
at the cold front may lead to the vertical motion and
.attendant reduction of moisture-bearing capacity which
is needed to produce rainfall. But in the case of many
cold fronts passing over Florida, it is found that the
rainfall occurs a hundred or more miles ahead of the
front; the frontal passage itself is often dry.
This displacement of the rain relative to the front
is explainable in terms of characteristic patterns of
vertical motion accompanying the meanderings of the
mid-latitude westerly current. The previously men-
tioned undulations of the westerly stream are called
"troughs" and "ridges" by meteorologists, in reference
to the corresponding configuration of the atmospheric
pressure field. The significance of these is that, in the
levels for which such motion is relevant in connection
with rainfall, troughs and low pressure centers are
characteristically accompanied by upward vertical
motion. The so-called "anticyclonic flow patterns"
which go along with ridges or centers of high pressure
are, on the other hand, typically connected with de-
scending motion, leading to warming and increased
moisture-bearing capacity of the air. The winter rain-
fall ahead of a cold front is associated with air flow
of the kind that occurs with a trough in the upper
current, called cyclonic, while anticyclonic conditions
often prevail at the front itself and almost invariably
in the cold air behind it. Cold-front rain tends to be
of brief duration (usually a few hours at most) and
of moderate to heavy intensity.
Another mode of interaction between the con-
trasting air masses of the middle latitudes in winter is
for the cold air to recede and be replaced by warm air
at the ground, often with active ascent of the warm
air along the gently sloping boundary separating it
from the denser cold air beneath. In the case of re-
treating cold air, the boundary is called a warm front.







11


The occurrence of warm fronts in the Florida area is
not as frequent as that of cold fronts. When they do
occur, however, they usually produce widespread pre-
cipitation of light to moderate intensity which some-
times lasts several days.
Most wintertime occasions of excessive accumula-
tions in a short period of time, such as the 24-hour
total of more than 8 inches at Pensacola in February
of this year, are found to be associated with a strong
northward flow of Gulf and Caribbean air. Often there
is no frontal surface near enough to be acting as a
barrier, and the rising motion is attributable entirely
to unstable thermodynamic characteristics of the atmo-
sphere and a wind field conducive to promotion of
rising currents.
Summer. In summer, the flow pattern of the water-
bearing layers is characterized by northward displace-
ment of the westerlies and establishment of two large
anticyclonic cells, one over the continent and one to
the east of the lower Atlantic coast. The former anti-
cyclone is particularly well pronounced at elevations
from about 10,000 to 20,000 ft. The latter anticy-
clone, which is a semipermanent feature of the sub-
tropical Atlantic ocean, moves northward and becomes
much intensified in the summer. It is strongly de-
veloped at the earth's surface as well as aloft, and the
southeasterly flow at its western end brings Florida
and most of the southeastern United States under
the influence of a deep stream of maritime tropical air.
Between the continental high-level anticyclonic
flow (normally found over the lower Mississippi Val-
ley) and the oceanic high-pressure cell, there is found
in the mean upper-air flow pattern over the Florida
peninsula a weak trough with its cyclonic type of air
motion.
The properties of the cyclonic flow associated with
a trough or low-pressure center are such that, even
though there may not be sufficient rising motion to
cool the air to saturation, the density stratification of
the atmosphere is changed toward a state of unstable
equilibrium with respect to vertical motion. The con-
verse is true of anticyclonic flow.
Droughts occur in conjunction with the establish-
ment of a strong anticyclone in a particular position
which is unfavorable with respect to precipitation in
the region. The summer of 1954, characterized by
drought in northern Florida and adjacent states, was
one in which the development and location of the
mean anticyclones were highly abnormal.
As the subtropical oceanic high moves northward,
the organized weather systems of the tropical easter-
lies become increasingly important as organizers of
the general tendency toward free vertical motion which
characterizes the Florida summer atmosphere when


not under anticyclonic influence. Excessive rainfalls
of summer are associated with the action of some or-
ganizing influence, generally of tropical type although
not necessarily of the extreme degree of development
(in terms of a low-pressure center) that is found in
tropical storms and hurricanes.
Except for the case of hurricanes, the summer rain
is essentially due entirely to showers and thunder-
storms; frontal rain is relatively unimportant. Thus
the rain tends to be spotty in time and space, partic-
ularly when no organizing system of the tropical easter-
lies is present. This spottiness presents acute problems
in precipitation measurement and forecasting, some
of which are indicated below.
Transitions. Autumn and spring are transition
periods between dominance of the westerlies (and
fronts) in winter and the various types of subtropical
circulations (and "local" showers and thunderstorms)
in summer. Generally these are periods of minimum
rainfall over Florida. They are the periods in which
the average large-scale circulation is closest to an ar-
rangement favorable to drought over the area.

Space variation of annual rainfall totals
Despite the lack of topographical variation in
Florida, there is considerable variability of rainfall
within fairly short distances even in long-period aver-
ages for the individual seasons. The combinations of
these within-seasons spatial variations with the dis-
tinctly differing types of spatial variations, character-
teristic of Florida's position in the transition zone be-
tween a middle-latitude continent and a tropical ocean,
give Florida a complicated distribution of annual rain-
fall totals.
In Table 1 are presented data on the annual rain-
fall at some of the Florida stations with reasonably
adequate records. The stations have been selected to
illustrate characteristics of interest for the purpose at
hand; since some stations are not well-known urban
centers, locations are shown in Figure 1. The state-
wide mean of annual rainfall is 53.5 inches. In Column
1 of Table 1 are shown the mean rainfall figures at
the selected stations for years of the period 1919-1953
having complete records.
We may observe that average rainfall ranges from
as much as 69 inches per year to as little as 48 or 49
inches. At a few locations on the peninsula not in-
cluded in the list, the average is in the neighborhood
of 45 inches. The Florida Keys, which generally have
decidedly smaller rainfall totals and present a separate
problem, are excluded from this study.
The main areas of high annual averages are in
the extreme northwest section of the state and at the
southern end of the peninsula. The highest amounts










are found at the inland stations of the extreme north-
west section, as exemplified by DeFuniak Springs.
Rainfalls averaging in excess of 60 inches per year are
found to the west of the eastern border of Alabama
and in the Dade-Broward-Palm Beach County area.
The portions of the state between these two geograph-
ical extremes exhibit a pattern of variation from rela-
tively dry areas, with rainfall averages on the order
of 48-50 inches, to relatively wet 55 or 56 inch totals.
While the stations have been chosen with an eye to
their representativity of the principal features, the data


cannot be regarded as a basis for any detailed conclu-
sions. The choice of the period of record for which the
means in Column 1 are computed is a compromise
among several desirable features: sufficient length of
record for stability, homogeneity of period of record
among stations, and strategic location. The number
of complete years of record used is given in Column 2.
Column 4 gives the median annual rainfalls for the
same locations. The significance of the median rainfall
is that, at a given station, 50 per cent of the years of
record had an annual total less than the median and


TABLE 1
Florida Annual Rainfall
Space distribution of mean and median rainfalls (inches) and
reliability measures.





S4 5 6 9 0 1 12

r 0
n P4 ss 2

1 2 3 4 5 6 7 8 9 10 11 12
Pensacola --- 62.6 35 1880-1953 59.7 59.7 78.0 42.3 18.3 17.4 35.7 60 18
DeFuniak Spr..... 69.2 29 1897-1953 66.8 67.6 86.6 55.0 19.8 11.8 31.6 47
Quincy ..... ------ 55.6 35 1917-1953 56.6 55.4 68.4 42.0 11.8 14.6 26.4 47 15
Apalachicola .---_... 56.5 35 1904-1953 55.3 56.9 75.6 47.0 20.3 8.3 28.6 52
Jacksonville .....------ 52.6 35 1867-1953 52.2 51.8 61.8 40.4 9.6 11.8 21.4 41 15
Federal Point ---.---- 55.4 29 1893-1953 53.0 53.9 64.3 42.9 11.3 10.1 21.4 40
New Smyrna ---.----- 49.5 27 1893-1953 45.3 48.8 64.1 35.8 18.8 9.5 28.3 62
Gainesville 52.5 35 1898-1947 49.3 50.3 61.0 40.8 11.7 8.5 20.2 41
Ocala --- 55.8 28 1892-1940 55.0 54.3 65.0 45.9 10.0 9.1 19.1 34
1944-1953
Orlando --- 52.9 34 1892-1943 51.8 51.7 62.6 39.8 10.8 11.0 22.8 43
Orlando Water P1. 51.3 26 1927-1953 51.6 51.3 60.8 39.6 9.2 12.0 21.2 41 13
Bartow ..... ------ 56.0 35 1896-1950 56.1 55.5 67.6 44.8 11.5 -11.3 22.8 41
St. Petersburg ....-----. 52.8 34 1915-1953 52.2 52.6 64.9 39.8 12.7 12.4 25.1 48 14
Punta Gorda .......--- 52.6 31 1915-1953 52.3 52.3 63.8 42.2 11.5 10.1 21.6 41
Moore Haven ------- 50.7 33 1919-1953 48.4 50.7 61.6 40.7 13.2 7.7 20.9 51
Fort Pierce .---- 53.3 34 1901-1950 51.9 52.4 69.5 40.5 17.6 11.4 29.0 56
Fort Lauderdale ..-- 64.0 28 1913-1953 62.0 62.5 80.4 47.0 18.4 15.0 33.4 53 18
Homestead ------- 63.3 28 1910-1953 64.0 62.8 74.7 51.8 10.7 12.2 22.9 36 13


*Location Changes
1) Gainesville
1898-7/1948 2838'N 8219'W (Gainesville No. 1)
8/1948-1953 29'39'N 8221'W (Gainesville Univ. No. 1)
2) Orlando
1892-1943 28"32'N 8122'W (Orlando No. 1)
1944-1949 2833'N 8120'W (Orlando Airport No. 2)
1950-1953 28'33'N 8120'W (Orlando Airport No. 1)
3) Bartow
1896-8/1951 2752'N 81'49'W (Bartow)
9/1951-1953 27"54'N 81"50'W (Bartow No. 1)
4) Fort Pierce
1901-1950 27'26'N 8020'W (Fort Pierce No. 1)
1951-1953 27"25'N 8019'W (Fort Pierce No. 2)


In the cases of Orlando and Fort Pierce, overlapping years
of record at the locations whose records had to be combined
indicate that the changes of location may have had a significant
effect. At Fort Pierce in 1950, the two locations had 56.69 inches
and 48.26 inches at No. 1 and No. 2 respectively. At Orlando,
1943 is an extreme example, with 49.40 at Orlando No. 1, and
39.10, 39.61 at Airport No. 1 and No. 2. The substitution of one
Airport station for the other in 1950 is less significant as expected;
however even here 1944 shows 47.79 at Airport No. 1 and 48.85 at
Airport No. 2.
For these stations, the means in Column 1 have been com-
puted from combined records. The data on medians and decile
limits are computed for records at one location, as indicated by
the periods of record in Column 3.


I _~






r'4


50 per cent equaled or exceeded it. The median is
a more useful statistic than the mean when dealing
with frequency distributions which are highly skewed.
Since frequency distributions of monthly totals tend
to be more highly skewed than those of annual totals,
it is especially suitable to use the median in dealing
with such distributions.
The values in Columns 4-6 of Table 1 were selected
from a summary of all known records of Florida rain-
fall through 1952, prepared by the Division of Water
Survey and Research of the Florida State Board of
Conservation.' The years of record upon which the
medians given in Table 1 are based are indicated in
Column 3, preceding the column of medians.
It can be seen that the total length of record varies
S widely among the stations listed. Such inhomogeneity
of periods of record is undesirable from the point of
view of the best climatological practice, but it can be
tolerated for most purposes of this summary. All
records are long enough to have achieved stability of
median and mean; and although a gradual swing
toward higher annual totals seems to exist in most
records which extend far enough back to justify look-
ing for it, the trend is not strong enough to merit con-
cern. To show the basis for the latter statement, the
means for the periods covered by the medians are pro-
vided in Column 5. Comparison of Columns 1 and 5
will show that, on the whole, the means for periods of
record going back to 1900 or earlier are 1 to 2 inches
less than the respective standard means computed
from 1919-1953 records. The criterion of homogeneity
of records is the basis for the cropping off of the years
earlier than 1919 in the above discussion of spatial
distribution of annual means and for certain of the
measures of variability introduced later, but lack of
homogeneity has been considered of negligible im-
portance in regard to the uses to be made of median
and decile distributions.

Reliability of annual rainfall totals
The difference between mean and median for the
same period of record can be used as a measure of the
skewness of the frequency distribution of yearly rain-
fall totals. For the stations given, these differences ex-
pressed as percentages of the means are less than 8
per cent, and hence must be judged as rather small.2
However, since the variability of rainfall about the
mean is a matter of prime interest in terms of water
resources, it is desirable for us to consider more il-
luminating measures of this property than the crude
indication of skewness.
Columns 6 and 7 of Table 1 present the rainfall
totals typical of very wet and very dry years, respec-
tively. The number in Column 6 gives the rainfall


amount which was equaled or exceeded in only one
year of ten for the period summarized (see Column 3)
and that in Column 7 gives the amount exceeded in
all but one year of ten. The succeeding pair of col-
umns gives the extent (in inches) to which these decile
values depart from the median value for each station.
It is apparent that this type of statistic gives one
useful indication of the range of reliable incidence
of yearly total rainfall.
The data in these columns, 8 and 9, show that
mostly (though not always) very wet years exceed the
median by more than very dry years fall below the
median. Departures on the wet side of the median
range up to 17 to 20 inches, with about 10 inches in
that direction showing at those stations with the most
year-to-year consistency. The 10 per cent of driest
years have departures below normal of less than 15
to 18 inches, while those stations which show the least
drop in very dry years depart at least 7 inches.
Columns 10 and 11 contain a further reduction of
the decile data in the preceding columns, giving the
contrast between relatively wet and relatively dry years
both as absolute contrast (in inches) and as percentage
of the station's median amount. The point of ex-
pressing the difference between the one-tenth and nine-
tenths decile amounts in terms of percentage of the
median is to indicate the importance of this spread
in relation to that annual total having 50-50 ex-
pectancy.





It will be recognized that this kind of statistic is
a contrast measure for occurrences of relatively small
expectancy. Another annual rainfall reliability mea-
sure which is sometimes offered is the "relative vari-
ability"3 or "percentage variability." It is defined as
the ratio (in per cent) of the sums of all deviations
from the mean, averaged without respect to sign, to
the mean. The value of this is presented in column
12 for a scattering of the stations in Table 1. The
period of years used in computing these numbers was
taken the same as for Column 1, for the same reasons.
We observe that the relative variability ranges from
13 to 18 per cent. With this small sample, there is no
particular significance to be ascribed to the array of
values. In the context of the world-wide distribution
of the statistic,4 it is found that there is some over-all
tendency for high relative variability to be correlated
with small annual rainfall (especially for annual
totals less than 30 inches), reflecting the fact that the
definition is not completely successful in producing
a measure of reliability alone, with no dependence on
the amount of precipitation.2 As far as the distribution
over the eastern United States is concerned, the annual
averages are large enough to permit comparisons of
relative variability among stations of differing yearly
sums. On the map in the cited reference,4 Florida is
shown as having 15 to 20 per cent relative variability,
with the United States east of the Mississippi River
having less than 20 per cent. Within the latter area
there is an extensive portion, encompassing much of
the Great Lakes region, the Ohio and Tennessee val-
leys, and New England, in which values of 10 to 15
per cent obtain. This means that the relative vari-
ability decreases with decreasing rainfall, and in gen-
eral the reliability of the annual rainfall over Florida
is less than that of the drier parts of the eastern United
States. However, the values of relative variability over
Florida are not large by any standard. The 44 per cent
found at Yuma, Arizona, is an example of the highest
values in the United States; however, here the effect
of having a small mean enters to detract from the
usefulness of relative variability as a measure of re-
liability alone.
There exists of course, a positive correlation be-
tween the relative variability and the wet-dry contrast
of Column 11; but relative variability naturally is
smaller and more uniform over the state. As will be
seen later, the relative variability over the years of
the totals for a given month show that the small values
of departures of annual totals from annual means are
not to be explained as due to repetition of the same
monthly pattern in year after year.


"17- ~ ~ -


Seasonal variation of rainfall
The most striking feature of Florida's rainfall
from the point of view of time variation is the domi-
nance of the months from June through September
in the over-all rainfall picture. These months account
for more than one-half of the average twelve-months
total.
Tables 2 and 3 contain data based on monthly
rainfall records1 at selected locations throughout the
state. For the purpose of condensing information on
seasonal variations into more convenient form, the
monthly medians are used in Table 2 in the form of
averages for seasonal groups of three months each.
Clearly, the average median for the months of a season
is in general not the same as the seasonal median.
However, the use of these average medians has the
advantage of evading the work of forming seasonal
medians by combination of the monthly data, while
still revealing most of the characteristics which are of
interest. Table 3 gives individual monthly medians
for the same stations to illustrate a few points which
are obscured by the combination into seasons.
Turning to an examination of the data in Column
1 of Table 2, one sees that the rainfall of the winter
months is greatest in the northwest and decreases
down the peninsula. The average values for December
to February are as high as 3.9 inches at Quincy. They
are more than 3 for the panhandle, between 2 and 3
over the northern one-third of the peninsula, and
under 2 from the latitude of Orlando southward. The
driest part of Florida in the winter months, from the
standpoint of rainfall equaled or exceeded in one-half
of the winter months, is in the southern interior of
the peninsula. The average median of 1.1 inches at
Moore Haven, on Lake Okeechobee's west coast,
typifies winter conditions in that region.
The pattern of average median rainfall for summer
months (June to August) is represented in Column
3. Here we find a general increase from northwestern
Florida toward the south-central peninsula, super-
imposed on a geographical distribution of great ir-
regularity. The panhandle values are generally be-
tween 5 and 6 inches per month, while the highest
values, up to 8 inches, are over the south-central and
western peninsula. It would be necessary to have a
much denser coverage of stations chosen with careful
attention to homogeneity of records to obtain a pic-
ture of the geographical distribution of summer rain-
fall which is reliable in detail. While it is not ad-
visable to generalize too freely about the rather com-
plex pattern that more detailed data reveal, it is
not amiss to point out the tendency for a relatively
dry strip along the east coast in the averages of June-










TABLE 2
Seasonal Variation of Rainfall
Averages of monthly median rainfalls (in inches) by seasons with
interseasonal contrasts of average medians and average seasonal
totals, for representative Florida Stations (based on period shown
in Column 3 of Table 1).


Average Monthly Rainfall Median
By Seasons


Winter
Dec.-Feb.

1

3.7
3.9
2.6
2.4
2.0
2.0
1.8
1.8
1.4
1.1
1.6
1.7
1.2


Spring
Mar.-May

2

4.2
4.4
2.7
2.6
2.4
2.7
2.7
2.2
2.4
3.1
3.1
3.8
3.6


Summer
Jun.-Aug.

3

5.8
5.8
6.0
6.7
5.5
7.7
7.0
7.0
7.4
7.0
5.3
6.3
8.0


August nionhl\ medians. This tends to smooth out
for the rain\ season as a whole because of high Sep-
tember rainfall on the lower east coast.
The secondary maximum in the seasonal march
of mronthl\ ainlall in part of the state is at least sug-
gested b\ the spi ing-months medians presented in
Column 2 ol Table 2, in that the panhandle stations
hale distinctly higher medians than elsewhere in the
state. This regional difference is primarily a mani-


Fall
Sep.-Nov.

4

3.5
2.6
3.9
4.3
3.9
3.6
3.7
3.8
3.8
3.6
5.1
6.1
6.3


Summer-Winter
Contrast


From
Average
Medians

5

+2.1
+1.9
+3.4
+4.2.
+3.5
+5.7
+5.1
+5.2
+6.0
+6.2
+3.7
+4.6
+6.8


From
Seasonal
Totals


Fall-Spring
Contrast


From
Average
Medians


7

-0.7
-1.8
+1.2
+1.7
+1.5
+1.3
+1.0
+1.6
+1.4
+0.5
+2.0
+2.3
+2.7


From
Seasonal
Totals

8

<-4
-1
+2
+2
+4
+3
+5
+4
>6
>6
>6
>6
>6


festation of the fact that the spring frontal interactions
of Gulf and continental air masses usually occur too
far north to affect the peninsula significantly. The
tracks of this kind of storm continue to affect greatly
the northern area in spring, being often more produc-
tive of precipitation then than in winter. The nature
of the spring maximum-minimum system as a minor
wiggle occurring from March to April is more evident
in Table 3.


TABLE 3
Course of Annual Rainfall Variations At Selected Stations
Entries are monthly medians, in inches.


SPensacola
Quinct
'Jackson lle
,.New Sni\rna
Ocala .
[Orlandi ..
1S. Peternbing
SPuna (,orda
Moore Hatuin
Fort Pieice
Fort Lauderdale
Homneltead


Pensacola
duincs
aackson ille
TFederal Point
New Smrna .
'Ocala
Orlando ...
St. Peterburg
Puma Gorda
Moore Hatent
Fort Pierce
SFort Laudeidale
',Home'lead


Range
of
Monthly
Medians

4.1
4.9
5.1
3.8
7.0
6.0
-7.0
6.5
6.9
6.0
7.0
8.8


*Months of highest medians are indicated by an asterisk.
tMonths of lowest medians are indicated by a dagger.


I I I









A secondary region of relatively high average spring
medians at the southeastern end of the peninsula has
a quite different origin, being the effect of the earlier
onset of the major rainy season on the southeastern
coast, from whence it progresses westward and north-
ward.
The data on average median monthly rainfalls
for fall, in Column 4 of Table 2, represent averages
for a time of rapid decrease from summer peaks to
late fall minima, and as such are not very meaningful
as indicators of the medians of individual months.
The geographical variations characteristic of the sea-
son are obscured by the averaging, even more so than
in the other transition period in spring, and it is
desirable to refer to the individual monthly medians
given in Table 3.
There is a very regular transition of the months
of lowest median rainfall, from October in the western
panhandle to November over the peninsula north-
west of the Moore Haven-Fort Pierce line, and finally
to December at those stations and southeastward. This
is an excellent index of the transition from a region
whose seasonal pattern is strongly influenced by winter
interaction of warm, moist Gulf air and cold air from
Canada or the Pacific to one where winter brings no
comparable substitute for the rain-producing con-
ditions in the tropical air that covers the southeastern
United States in summer.
It is interesting to note the gradient of seasonal
variation between the geographical extremities of the
state in terms of the interseasonal contrasts, summer
minus winter and fall minus spring. The average
medians by seasons are useful for this purpose. The
excess of summer precipitation over winter is ex-
hibited in terms of differences of average medians in
Column 5 of Table 2. All stations, it may be observed,
have a summer maximum of precipitation. Compared
to winter, summer has a rainfall excess in terms of
average medians on the order of 2 inches in the ex-
treme northwest, increasing to 4 to 5 inches over the
central peninsula and to 6.8 inches at Homestead.
When the differences are plotted on a map, it becomes
apparent that the smaller summer excesses characteriz-
ing the panhandle extend across the northern part
of the peninsula and far down the east coast. The
latter feature would not be as marked were it taken
into account that for much of the state, particularly
the lower peninsula and east coast, September is a.
summer month as far as classification of seasons by
rainfall is concerned.
In fact, the highest monthly medians of rainfall for
the stations southward from Lake Okeechobee occur
in September, with an associated secondary maximum
in June or July. One interpretation of this tendency


toward a double maximum in the rainy season, one
early and one late, is the systematic shift of an upper-
air flow pattern favoring occurrence of atmospheric
convection." The favored locale of this pattern moves
across the peninsula from east to west as the large-scale
upper-air circulation pattern of summer becomes es-
tablished, and back again as the season wanes.
This explanation provides a reasonable unification
of the summer rainfall variations over the lower
peninsula and a portion of the east coast. However,
the summertime Florida peninsula does not act as
a unit in its average variations through the rainy sea-
son, and it is apparent that the explanation of the
differences involves consideration of other influences
not homogeneous over the region. In this respect,
it may be recalled also that the lower peninsula and
east coast, toward late summer, experience an in-
creased incidence of weather disturbances which ar-
rive from the Caribbean in varying degrees of organi-
zation and intensity, up to hurricanes.
Of the stations listed in Tables 2 and 3, only Fort
Pierce has a simple seasonal variation of single sum-
mer maximum and single winter minimum. In ad-
dition to the double summer maximum which has
been pointed out for the lower peninsula and east
coast, and is also found again on the upper east coast,
there is a tendency for occurrence of a secondary
maximum in February or March at many stations. It
is particularly marked over the panhandle, but can be
traced far down the peninsula, disappearing at the
stations from Moore Haven and Fort Pierce southeast-
ward.
The differences of average fall monthly medians
and those of spring, given in Column 7, indicate quite
well the change from an excess of spring rainfall over
fall rainfall at the northwestern tip to a pronounced
fall excess over the southeastern peninsula.
Columns 6 and 8 have been included in Table 2
to give an idea of the differences (in inches), summer-
winter and fall-spring, in terms of mean seasonal
totals. The data have been interpolated from pub-
lished maps6 which were based upon the half-century
ending in 1938. At that time there was a sufficiently
large number of stations with records of acceptable
length, so that we may confidently consider the figures
thus obtained to be a fair representation of existing
conditions.
In the original map of summer-winter contrast
there appears a large region of winter excess over the
Mississippi Valley and western Gulf states, with a
center of 4-inch winter excess over northern Louisiana.
This has changed to a summer excess of 3 inches by
the time one shifts to extreme northwestern Florida.
The summer excess increases rapidly eastward over


rl -- -. 1























1


Florida, becoming more than 10 inches over the area
eastward of Tallahassee and more than 15 inches in
central and western portions of the southern two-
thirds of the peninsula.
The differences of fall total minus spring total,
given in column 9, show a spring excess of 4 inches
over the western tip of the panhandle, changing to
a fall excess east of Tallahassee. The fall excess in-
creases rapidly to values of more than 6 inches over
approximately the southern half of the peninsula.

Reliability of monthly totals of rainfall
The reliability of individual monthly rainfall
totals at four stations is presented in Table 4 in
terms of relative variability. The four stations used
are among those for which relative variability of an-
nual rainfall is given in Table 1. These four have
been selected on the basis of geographical dispersion.
Recalling the definition of relative variability to be
the ratio of the average absolute value of departures
from the mean (in this case for individual months,
e.g. all Januaries of the period of record, etc.), to the
mean, it is not surprising that there should be some
tendency toward an inverse relationship between rela-
tive variability and the mean.
The extent to which relative variability of monthly
rainfall totals can be expected to be free of dependence
upon the mean has to our knowledge not been studied,
but it is certainly valid to draw conclusions as to space
and time differences in reliability when the means
are about the same. When one examines the data
on this basis, one observes a striking tendency for


TABLE 4
Reliability of Monthly Totals of Rainfall
Mean monthly totals of rainfall (inches) and relative variability (percent) at four
Florida stations with records from 1919 through 1953*


Pensacola


Mean


Relative
Variability


Jacksonville


Mean


Relative
Variability


St. Petersburg


Mean


January --- ----- 4.4 52 2.6 53 2.4
February ....-------------..-- 4.4 57 2.9 46 2.8
March ---- 5.8 49 3.4 58 3.1
April 5.3 58 3.0 49 2.9
May .----.......-------- 4.3 55 3.8 52 2.7
June ---- 5.1 58 6.7 43 5.8
July ..----.-..... _..---- --- 7.5 37 7.6 30 9.2
August 7.8 50 6.6 38 8.4
September ........--------... 6.1 65 6.9 44 8.2
October ----- 3.7 70 4.9 68 3.5
November --- 3.5 78 1.7 75 1.6
December .. ------ 4.4 48 2.6 61 2.2


Relative
Variability
61
69
56
73
57
40
36
64
37
57
85
65


Homestead


Mean
1.6
1.7
2.1
3.4
6.4
8.1
8.4
8.4
10.0
9.1
2.1
1.2


Relative
Variability
86
82
74
60
54
55
27
52
50
51
84
67


*At all stations except Homestead the number of years of record for each month is the full 35 possible for the period. At Home-
stead, no month has fewer than 31 years in the period, three have 31, two have 34.


higher reliability at any given station in July than in
other months of equal or higher means. The same
kind of significant differences in reliability can be
found among the values for certain given months at
different stations. For example, the August rainfall
at St. Petersburg is clearly less reliable than August
rainfall at the other stations.
In reference to a remark made in the discussion oi
reliability of annual rainfall, it is to be noted that the
relative variability of the rainfall of individual months
is three or four times as large as the relative variability
of annual totals. This is at least partly an expression
of the fact that although the timing of seasonal varia-
tions may differ from one year to another, the major
controls do operate in pretty much the same sequence
and to the same extent in every year. For example,
the summer rainy season may start early or late, but
almost invariably encompasses the month of July.
Hence July is not much affected by earliness or lateness
of summer rain, while departures from the mean in
fringe months like May or September will tend to be
strongly affected, but in opposite directions from one
another.
The foregoing brief discussion of the variation from
year to year of the rain accumulations for individual
months suggests that there are important aberrations
which cannot be ignored in regard to reliability of
replenishment of water resources by rainfall. It is not
within our scope to enter into any further analysis of
the reliability measures. The range of values taken
and other significant aspects can be noted by the
reader as may suit his purpose.









TABLE 5
Spatial Variability of Rainfall
(a)
1943 rainfall (inches) at three rain gage locations in Orlando area

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Total
........ 1.61 0.57 4.52 1.60 4.83 3.66 9.08 7.50 11.66 2.56 0.77 1.04 49.40
_.. 1.71 0.39 4.51 1.45 4.76 7.07 8.60 7.69 11.74 4.07 1.08 1.22 54.29
_.... 1.22 0.45 3.71 1.53 5.36 3.60 5.08 5.64 7.90 2.59 0.94 1.08 39.10


Orlando No. 1
Orlando Water Plant
Orlando Airport No. 1


28*32'N 81*22'W
2833'N 8121'W
28'33'N 81020'W


(b)
Differences between yearly totals of Orlando Water Plant and Orlando Airport No. 1
(Water Plant minus Airport No. 1) for five years of concurrent records.


Year 1949
Difference -2.18


1950
-7.94


1951
+1.06


1952
+4.88


1953
+8.08


Spatial variability

However, in the interpretation of rainfall amounts
and variability measures, it must be recognized that
there may be considerable differences between places
a small distance apart. An excellent example of this
is found in the rainfall records of Orlando, Florida.
Table 5 (a) shows the records for 1943 at three stations
whose locations are given in the table. The greatest
distance between any two stations is two nautical
miles, according to the locations given in the records.
The 1943 totals differ by no less than 9 per cent of the
means for the locations of any gages, and by 30 per
cent for the Water Plant gage and the current U. S.
Weather Bureau location. To show that this year,
though exceptional, is not an isolated occurrence,
Table 5 (b) gives the differences between the annual
totals at the two currently active recorders for the
years 1949-1953 inclusive. The total absolute dif-
ference is 25 inches, although the cumulative alge-
braic difference averages out to less than an inch a
year. Small-scale differences of this sort can be of
considerable significance in areas characterized by
shower and thunderstorm precipitation.

Role of thunderstorms

The thunderstorm7 8 produces intense rainfall of
short duration and limited extent. The type with
which Floridians are familiar is known as the "local"
thunderstorm, in accordance with the characteristic
of being productive of relatively small and distinct
rain areas. The conditions conducive to the develop-
ment of such storms are extremely well developed over
Florida, particularly the peninsula, in summer. Out-


side the period from about June through September,
Florida does not have any remarkably higher occur-
rence of thunderstorms than other parts of the United
States. The thunderstorms of those other months are
associated with frontal activity, and generally are im-
bedded in widespread areas of lighter and more per-
sistent rain.
The occurrence of thunderstorms is invariably as-
sociated with high atmospheric moisture content and
a thermodynamic condition of the atmosphere such
that vertical motions, once induced, are favored and
accelerated. In summer, the northward shift of the
Atlantic tropical easterlies supplies this area with air
having a long history of addition of heat and moisture
from below. The combination of this general situation
with the greatly increased response of the land-surface
temperature to solar heating produces a summer
atmosphere over the peninsula in which thunderstorm
rainfall accounts for virtually all of the total rainfall.
Thus, for the months June through August, most
of the stations in the state experience an incidence
of thunderstorms which averages one day of every
two. In the peak month of July, the number of days
on which thunderstorms occur averages 4 out of every
5 in many locations. The center of maximum occur-
rence lies over the western half of the peninsula, from
Punta Gorda to Orlando. The dependence of the local
type of thunderstorm on heating of the ground ac-
counts for a pronounced maximum of incidence in
hours from noon to 6 P.M., except in the southeastern
coastal section. At Miami, only a slightly higher per-
centage of the total occurrence is found between 12
noon and 6 P.M. than from 6 A.M. to noon. The
morning accounts for about 30 per cent, the afternoon


Orlando No. 1
Water Plant -
Airport No. 1 .


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40 per cent, and evening about 20 per cent. This may
be contrasted with the situation at Tampa and Jack-
sonville, where the afternoon accounts for 65 to 70
per cent of all summer thunderstorm occurrences,
against 15 per cent in each of the adjoining 6-hour
intervals.
The duration of a thunderstorm is short, and one
a day is generally par; so that in the season, rain falls
only 6 to 7 per cent of the time. The space vari-
ability is very large, as can be seen by the large contri-
bution of the summer months to the variability in the
Orlando area shown in Table 5 (a).
Against the background of broad-scale upper-level
circulational features and thermodynamic properties
of tropical air which are involved in the phenomenon,
there are various possible secondary factors operating
to give the Florida peninsula a lion's share of thunder-
storms over the eastern United States. The mean
trough between the subtropical oceanic and conti-
nental anticyclones has been suggested as an important
control. Another suggestion" concerns the opposing
directions of the afternoon sea-breeze effects on the


parallel east and west coasts, acting to promote vertical
currents.
Large and seemingly irregular variations of summer
rainfall can be found when the space and time distri-
butions are examined in detail, indicating that there
is considerable inhomogeneity in the factors at work.
Observers in Miami area have found that distinct
patterns of summer showers are discernible, and are
seeking to relate these to local factors for improvement
of forecasting accuracy.10 The Department of Meteor-
ology at Florida State University is engaged in an
investigation of rainfall distribution over the Florida
peninsula on summer days, primarily in an effort
to detect and evaluate tendencies toward organized
precipitation patterns and sequences in terms of the
large- and small-scale meteorological factors. This
work is being carried on with support from the Office
of Naval Research. It is hoped that this study will
be productive of an increased understanding of the
summer rainfall regime of the State of Florida and
will lead to increased ability to forecast the space and
time patterns of summer rains.


REFERENCES


1. State of Florida, State Board of Conservation, Division of
Water Survey and Research, 1954: Summary of observed
rainfall on Florida to 31 December 1952. Water Survey and
Research Papers, No. 11, 334 pp.
2. Conrad V., and L. W. Pollak, 1950: Methods in climatology.
(2nd ed.), Cambridge, Harvard University Press, 459 pp.
3. Conrad V., 1941: The variability of precipitation. Monthly
Weather Review, 69, 5-11.
4. Biel, E. R., Fig. 3.15 in Riehl, H., 1954: Tropical meteor-
ology. New York, McGraw-Hill, 392 pp.
5. Riehl, H., 1947: Subtropical flow patterns in summer. Dept.
Meteorology, Univ. Chicago, Miscellaneous Reports, No. 22,
64 pp.


6. Visher, S., 1943: Novel American climatic maps and their
implications. Monthly Weather Review, 71, 81-97.
7. U. S. Weather Bureau, Hydrometeorological Section, 1945:
Thunderstorm rainfall, Part I (figures). Hydrometeorologi-
cal Reports, No. 5, 155 pp.
8. U. S. Weather Bureau, Climatological Services Division,
1952: Mean number of thunderstorm days in the United
States. Technical Papers, No. 19, 22 pp.
9. Byers, H. R., and H. R. Rodebush, 1948: Causes of thunder-
storms of the Florida peninsula. Journal of Meteorology, 5,
275-280.
10. Gentry, R. C., and P. L. Moore, 1954: Relation of local and
general wind interaction near the sea coast to time and
location of air mass showers. Journal of Meteorology, 11,
507-511.




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