SELECTED CHARACTERISTICS OF PRECIPITATION
AT LONG BEACH， CALIFORNIA
John C． KIMURA and Gary L． PETERS
INTRODUCTION
Long Beach， California， is included with the Mediterranean Dry−Summer Subtropica1
（Csa）climatic type． The city is situated at 33°46’ North Latitude and l l 8°12’West Longi・
tude． Despite its coastal position， the region is included within the warm summer phase of
the Mediterranean climatic type because of the east・west coastal trend．Accordingly， the sea−
breeze does not counteract the effects of solar heat substantially to cool the area in summer．
Similar conditions are noted for other regions with an east・west coastal orientation．
Since Long Beach is situated in the southern extremity of the climatic region， average
annual precipitation is on the low range for this climate type． Although there are variations
in the average annual rainfall because of topography， coastal Southern California receives
between 10 and 15 inches（250−375 mm）． This rain is heav皿y concentrated during the
winter half of the year．
Summers are dry at Long Beach because of the subsidence of air that is associated with
the Pacific Subtropical Anticyclone． This high pressure cell has strengthened and has mi・
grated to a more northerly position and dominates the weather along the entire Pacific coast
of the United States by effectively steering Pacific storms away from the California coast．
Summer precipitation is primarily of convectional origin． This type is rare because of the
dominance of the Pacific Subtropical High． However， when this High weakens or becomes
displaced， moist tropical air from the Gulf of Mexico， Gulf of California， or the tropical
eastern Pacific Ocean may invade this region． Thunderstorms may then result． Tropical
storms are even more rare than convectional storms because of the dominance of the afore−
mentioned Pacific High， and， in addition， because of the effect of the cold water of the
California Current．
Winter storms that affect coastal Southern California are frontal． These storms advance
from the northwest as cold fronts as the Pacific High weakens and migrates to the south．
Along this coast 90％of the annual precipitation occurs from November through April， and
over 50％is recorded in the three winter months of December，January and February．
Six years ago California State University， Long Beach， received a National Science Foun・
dation matching grant fbr the purpose of establishing a weather station． The station has been
in operation fbr about five years and is maintained by the Geography Department． The sta・
tion is about 75 feet（23 meters）above sea・level and is on the roof of a three−story building．
This paper deals primarily with various aspects ofprecipitation as recorded by the automatic
rain gauge at the station， and in particular with rainfall intensity．Observations from the rain・
fall year（July−June）1971−72 through 1975−76 were analysed． During this period the
equipment was under repair for four of the months， unfbrtunately during the rain season．
Because the station has been in operation for only a few years， Long Beach Airport means
and extremes as reported in the Department ofCommerce publication，Climatography of the
一35＿
United States No．60−4（1970）， were utilized where necessary． Long Beach Airport is situ−
ated apProximately 31／2 miles（6 kilometers）from the university．
AVERAGE ANNUAL PRECIPITATION
The average annual precipitation at the Long Beach Airport is 9．85 inches． de Violini
（1974）suggests that the average annual amount is misleading as a measure of estimating the
annual expected rain in a low rainfall climate like that of Southern California，and that the
figure is biased toward the high side． He advocates the use of the median value in place of
the average annual figure．To obtain the median for the 30・year period 1931−1960，Depart・
ment of Commerce publications， Climatography of the United States nos．11−4（1953）and
86−4（1964）were consulted． Because data for the Long Beach Airport were not available，
figures fbr Downtown Long Beach were used． The median fbr this period for Downtown
Long Beach is 11．53 inches compared with a long−term average of 13．00 inches． The median
is 1．47 inches（about l l％）less than the mean． This implies that the annual expected precipi・
tation for the Long Beach Airport is 8．74 inches．
In regions where the average annual rainfall is low， the average number of rainy days 1ike・
wise tends to be low． Futhermore， the pbrcentage of the mean annual amount occurring in
short periods then tends to be higher． For instance，the Imperial Valley receives over 40％of
its annual precipitation on the wettest single day． In the state of California， Department of
Water Resources publication（1972）， an interesting series of maps depicts short−period rain−
fall as a ratio of the annual average values． The publication shows Long Beach as receiving
2．6％of its precipitation in the wettest oneくluarter hour；6．1％of the annual precipitation is
recorded during the wettest one hour；in the wettest 6・hour period，11．5％of the annual
normal is received；and， nearly 20％of the annual rain is received in the wettest 24−hour
period．
MONTHLY MEANS AND EXTREMES
Monthly rainfall normals fbr Long Beach Airport were used in lieu of those for the
University weather station in analysing seasonality of precipitation（Table 1）． The findings
revealed that the months of June， July and August have an average of O．07 inches． This
represents less than 1％of the average annual precipitation． The values increase f6r the
months of September， October and November to 1．26 inches and nearly 13％． However，
November alone is the greatest contributor to this period． November has an average rainfall
of 1．03 inches， which represents about 10％． This means that the percentage of rainfall for
September and October combined is just slightly rnore than 2％． The winter months of
December， January and February are the rainiest．During this period the total average is 6．27
inches， and in these three months nearly 64％of the annual rain is recorded at the Long
Beach Airport． Diminished amounts are noted fbr March， April and May． However， these
three months have 2．25 inches of rain， which represents approximately 23％of the annual
tota1． March alone has 1．37 inches，which represents about 14％of the yearly tota1．The fbre・
going shows that the rainiest months are from November through March， when approxi−
mately 88％of the total annual precipitation is recorded at the Long Beach Airport．Conroy
（1933）made a similar study of the precipitation distribution by decade periods fbr Los
Angeles and San Diego from 1887 to 1927．
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Table l Frequencies（percentages）of observed rainfall at California S tate University，
Long Beach，1）y categories of intensity＊（July，1971−June，1976）・
Rainfall Intensity
Average
Month
@ Monthly
orecipitation＊＊
Light（％）
July
0．Ol
0
`ugust
reptember
nctober
n．03
@0
movember
cecember
P．03
ianuary
eebruary
P．99
O．06
U（100）
O．17
larch
P．37
`pril
O．77
lay
O．ll
Q0（77）
P3（45）
Q7（54）
P6（52）
V9（65）
T9（54）
R5（66）
R（75）
iune
n．03
V（100）
sOTAL
X．85
P．97
Q．31
Medium（％）
Heavy（％）
0
P（100）
@0
U（23）
撃戟i38）
P9（38）
P2（39）
R8（31）
S7（43）
P7（32）
P（25）
@0
＊Rainfall intensity categories are as foUows：Light＝0．01 to O．04 inches， Medium＝0．05 to
O．20inches， and Heavy＝0．210r more inches．0．Ol inches＝0．254 mm；lmm＝0．039 inches
＊＊Long Beach Airport
Measurable rain（one−onehundredth of an inch or greater）occurs on an average of only
27days annually．Highest frequencies are for the three winter months of December， January
and February with 13 days， nearly half of the total annual rainy days；however， compared
with the total number of days， only a few have precipitation． March， likewise， has a high
frequency with 5 days， and November shows 3．Therefore，November through March has 21
rainy days compared to the average annual number of 27 precipitation days．
Precipitation variabiity from the annual average is high， and monthly variation likewise
has extrenies． While the average monthly values show at least some precipitation for every
month of the year， at the same time no recorded rain occurred for each and every month
sometime during the existence 6f the Long Beach Airport weather station． The rainy winter
months are no exceptions． On the other hand， far greater values than normal f6r individual
months， too， have been recorded． While the deviation in measurable amount is small for the
summer months（0．52 inches was recorded in June，1963）， the absolute percentage variation
is high． The June，1963， amount represents a positive variation of over 1，700％」n Septem・
ber，1961，1．31 inches of rain were recorded at the Long Beach Airport． This figure repre−
sents a positive deviation that is greater than 2，200％． The greatest deviation in terms of
レ
amount occurred in January，1969， when 11．24 inches were recorded． This is a positive
deviation of nearly 600％compared with the normal of 1．99 inches． In January，1969， a
series of frontal storms that originated at relatively low latitudes advanced on the California
coast．
CHARACTERISTICS OF RAINFALL
Table l illustrates two characteristics of precipitation in Long Beach． First， the seasonal
distribution of precipitation， from a minimum in July to a maximum in February， is clearly
37
discernible， though the data give frequency of rainfall intensities rather than actual amounts．
This patterh is perceived despite the fact that in the five years that the University weather
station has been in operation the rain gauge was under repair once during November， twice
in December， and on another occasion in January． This means that f6ur of the sixty data・
base months have been lost．Second， the data show that，for any season， most recorded rain一
飴11is in the light category， while very little、rain occurs in the heavy category． Most of the
precipitation results from frontal storms． These storms advance from the northwest， and
usually only the trailing edge of the cold front affects Southern California． Accordingly，
these fronts pass through with extremely light rain， or in the dry state． The rainfall fre・
quencies in the light and medium categories both vary from month to month， but it is
particularly interesting to note the changes in frequency of medium rain as a percentage of
total rainfall observations．
Table l also provides an indication of the changing contribution of medium rainfall to the
total rain as we progress from October to January． Of these data， only February does not fit
the overall pattern． Further data collection may show that February is not actually an
anomaly but， rather， the small sample data failed to accurately demonstrate the pattern．
The overall seasonal pattern of rainfall frequencies， as well as monthly variations in the
frequency categories， raises a broader question about whether the seasonal patterns of
observed rainfall intensities differ significantly in a statistical sense． Data here are aggregated
into three・month‘‘seasons”as fbllows：1）June， July and August（JJA）；2）September，
October and November（SON）；3）December， January and February（DJF）；and 4）March，
April and May（MAM）． For comparative purposes， the June， July and August period is not
used because of the low frequency in which rain was recorded．
Chi・squared tests are used to determine whether there are statistically different patterns
of observed rainfall intensities between pairs of‘‘season”categories as follows：1）SON and
DJF，2）DJF and MAM，and 3）SON and MAM．
Data fbr the three chi−squared tests appear in Tables 2，3and 4． The associated chi・
Table 2 RainfaU frequencies for September， Octo・
ber， November versus December， January，
February．
Rainfall Intensity
Season
Light
Medium
SON
39
17
cJF
P22
U9
Heavy
512
Chi−squared＝2．98
Table 3 RainfaU frequencies for December， Janua− Table 4 Rainfall frequencies for September， Octo−
ry， February versus March， April， May． 1）er， November versus March， April， May．
Rainfall Intensity
Rainfall Intensity
Season
Light
Medium
Heavy
DJF
122
69
12
lAM
X7
U5
S
Season
Chi−squared＝2．16
Light
Medium
SON
39
17
lAM
X7
U5
Chi−squared＝7．50
38
Heavy
54
squared values for the above table are 2．98，2．16and 7．50 respectively． At the o．051evel of
significance the critical value of chi−squared is 5．99． In order to establish a significant dif・
ference between two seasonal patterns the observed chi−squared value must exceed 5．99．
This means that only one pair of seasons（Table 4）shows a statistically significant difference
between the丘equency distributions of rain鉛11s．
No significant differences are observed fbr the data in Tables 2 and 3．This indicates that
the fall rainfall pattern（SON）does not differ significantly丘om the winter pattern（DJF）
and that the spring pattern（MAM）does not differ significantly丘om the winter pattern．
However， when the spring pattern is compared directly with the fall pattern a statistically
different pattern of rainfhll intensities is observed． This suggests that， though the seasonal
progression of rainfall intensity patterns gradually changes from September to May， signifi・
cant differences appear only when seasonal extremes are compared． In the fall there are few
storms and these storms are concentrated toward the end of the period as the Pacific Sub−
tropical High weakens and moves southward． On the other hand， in the spring storms are
more frequent and are producing proportionately more rainfall in the medium category．
CONCLUSION
From this brief study of rainfall seasonality and intensity at Long Beach， several conclu−
sions emerge． First， when rainfa皿intensity is recorded as light， medium and heavy， the
majority of rainfall observations are in the light category． The percentage of observations in
the light category vary considerably， however， from month to month， and ranges from 45
percent of all observations in November to lOO percent of all observations in both June and
September． Second，on the average there are relatively few days with rainfall during the year．
Third， there is a low average annual rainfall in Long Beach， even f6r a Mediterranean climate．
There is an obvious need for more data on rainfall intensity for this station as well as for
other stations． Seasonal variations in the pattern of rainfall intensity are intriguing． Further
data may well support the findings of this study． Similar studies for other climate stations
would be useful for comparative purposes． Once seasonal differences in rainfall intensity
patterns are well established there remains the task of developing and testing explanatory
hypotheses．
REFERENCES
California， State of， Department of Water Resources，（1972）． Californ ia C7imatic Variation， Sacramento，
18PP。
Conroy， C。（1933）． The Relative Distribution of Early and Late Season Rainfall in Southem California，
Mon th ly”lea th er R evゴεw，61， No．1， pp．15−16．
Department of Commerce， Envilonmental Science Service Administration，（1966）．ハ「ormal Mo〃’配yハxum−
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States， Technical Paper No．57，Washington， D． C．52pp．
Department of Commerce， En嘘onmental Science Service Administration，（1970）．α〃nates of the States，
Climate of California， Climatography of the United States， No．60−4，Silver Spring Md．，57pp．
Department of Commerce， National Oceanic and Atmospheric Administration，1）aily Weather Maps，
Weekly Series， Washington， D． C．
Department of Commerce， Weather Bureau，（1973）． aimatic Summaり20f the United States，（2zlifornia，
39
Supplement／br 1931 through 1952， Cljmatography of the United States No．1レ4，Washington， D．C．，
154pp．
Department of Commerce， Weather Bureau，（1964）．α伽〃’o Summary of the乙「nited States， Clalifornia，
5吻ρ’em ent for 1951 th rough 1960， Climatography of the United States， No．86−4， Washington，
D．C．，214 pp．
Department of Commerce， Weather Bureau，（1965）． Meteorological Summ〃ゴes Pertゴηeη”o．4 tmospherゴe
Tra〃sport and 1）ispersion Over Southern California， Technical Paper No．54， Washington， D．C．，86 pp．
de Violini， R．，（1974）．α脚’ic Handbook for、Point Mugu and Skenハ「icolas」rsland，」Part 1， Surface Data，
Pacific Missile Range， Point Mugu， California，139 pp．
一40一