218 
PACIFIC SCIENCE, Vol. Ill, July, 1949 
Mordy and Leopold (MS.) found highly sig- 
nificant correlation between trade wind rainfall 
on Oahu and the temperature at 300 mb. (about 
30,000 feet msl.) on the Honolulu sounding. 
This is particularly interesting because trade 
wind rain falls from clouds whose tops ordinarily 
do not exceed 10,000 feet msl. Recent studies 
by the authors indicate that the 300 mb. chart 
is probably the most important tool in fore- 
casting rainfall in the Hawaiian area. 
GEOGRAPHIC DISTRIBUTION OF RAINFALL 
Mean annual rainfall maps for the various 
islands in the Hawaiian group, which improve 
on the Oahu annual map of Voorhees (1928), 
are available In the Territorial Planning Board 
Report (1939). The only monthly isohyetal 
maps available are median rainfall maps for 
Oahu prepared by Halstead and Leopold 
( 1948), based on an 11-year period, 1936-1946. 
Tiillman (1936), in a study of rainfall dis- 
tribution over the central and western Pacific, 
prepared mean annual rainfall maps for the Ha- 
waiian islands of Kauai, Oahu, Maui, and Ha- 
waii. His monthly and annual means were gen- 
erally based on the 20-year period from 1905 to 
1924. The records of those stations which did 
not coincide with that base period were adjusted. 
His data came from the published records in 
Climatological Data. Tiillman did the work in 
Germany and apparently did not have access to 
Voorhees’ map. 
Tiillman’s isohyetal maps are sketchy, but his 
graphs of the annual march of rainfall at various 
stations are of interest. The mean rainfall for 
each month was plotted on a time-versus-preci- 
pitation graph for each station having the re- 
quisite records for direct use or adjustment to 
the 20 -year base. 
The Weather Bureau, in the publication 
Climatological Data , groups the stations of each 
island in geographic units selected primarily to 
divide windward from leeward stations. Oahu, 
for example, is divided into six areas. Using the 
stations within each of these geographic units, 
Tiillman compared qualitatively the curves 
showing the annual march of rainfall at each 
station and grouped together the stations which 
had similar curves. This provided two or three 
groups within each geographic unit. Those sta- 
tions lying in the same elevation zone had 
similar annual march curves. 
It was unfortunate that Tiillman did not 
discard the arbitary geographic units of the 
Weather Bureau and lump all stations having 
similar annual march curves. He would thus 
have been able to delineate, on various islands, 
zones which are similar in distribution of rain- 
fall during the year. This would be one indica- 
tion of similarity in rainfall processes. In a 
single given month there are large differences 
between the mean rainfall for each island; | 
moreover, during a given storm, .geographic 
units having apparently similar exposure may 
record widely different rainfall amounts. The 
problem of determining areas of similar rain- 
fall processes is, therefore, not as simple as 
might be expected. Delineation of such areas 
is important In the problem of developing 
techniques for both short- and longer-term 
quantitative rainfall forecasts for various por- i 
tions of each island. 
Tiillman’s group curves can be analyzed and 
consolidated to provide the recapitulation men- 
tioned. Only moderate accuracy could be ex- 
pected, however, because he did not publish 
the numerical rainfall data in tabular form 
with the curves. Quantities could be determined i 
only as accurately as the small-scale curves i 
could be read. 
The most striking features of the annual 
march curves are the exceptionally low mean j 
rainfalls for certain months. All groups of sta- 
tions for Kauai show a low rainfall in February, 
as low as, and in some cases lower than, the ; 
mean for June. Yet January and March tend 
to be among the months of highest rainfall. This j 
depression in February means is apparent to a 
lesser extent in all curves for Oahu, in those for I 
the northeast slopes of west and east Maui, 
and for the northeast coast of Hawaii. 
Radical increase in rainfall in November over 
October appears on the northeast coast of Ha- 
