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PACIFIC SCIENCE, VoL III, July, 1949 
the different elevations of maximum rainfall to 
convergence resulting from horizontal splitting 
of the trade wind by the higher mountains. 
It is stated in the Planning Board Report 
(1939) that only in the Kona area (western 
portion of Hawaii) does the summer rainfall 
(May to October) exceed that of winter. How- 
ever, Tiillman’s curves, checked by current in- 
vestigations of the authors, have disclosed that 
some of the wetter areas on Oahu and Hawaii 
have summertime maxima. 
Correlations between the wind direction aloft 
(10,000 feet) and the geographic distribution 
of rainfall were made by Wallen and Yeh 
(MS.). Though their conclusions are tentative, 
they indicated that relative to the direction of 
wind at 10,000 feet the rainfall maxima ap- 
peared on the lee side of islands and the minima 
on the windward sides. Though this at first 
seems contrary to the expected orographic 
effects, Wallen and Yeh present a qualitative 
picture of distribution of convergence and di- 
vergence which might explain the observed 
rainfall pattern. 
Riehl (MS.) studied 10 years of daily rain- 
fall records at 21 rain gages in north central 
Oahu. He delineated four districts which ap- 
proximately coincide with the areas known 
locally as Waimea, Waialua, Wahiawa, and 
Helemano. At the outset he defined "effective” 
precipitation as rain of 0.10 inches in a day at 
a given station. In summer 20 per cent of the 
total rainfall occurs in showers of less than 0.10 
inches in a day and is, therefore, ineffective. In 
winter only a negligible amount is ineffective. 
Of the total number of days of rain, 40 per cent 
in winter and 60 per cent in summer are in- 
effective. For the remainder of his study, only 
"effective” rain was considered. He then defined 
a "rainstorm” as one which "in each of the four 
districts half or more than half of the stations 
received [effective} precipitation.” He con- 
cluded that 80 to 90 per cent of the total pre- 
cipitation occurs in "rainstorms.” 
Isohyetal patterns of mean monthly rainfall 
due to "rainstorms” show higher rainfall over 
the mountains than over the plains. In other 
words, orographic lifting is an important factor 
determining rainfall distribution in general 
storms. 
Riehl found that rainstorm precipitation is 
more evenly distributed from mountain to plain 
in summer than in winter. 
MONTHLY AND ANNUAL VARIATIONS IN 
RAINFALL 
Variations in yearly values of rainfall for 
Oahu stations were studied by Nakamura 
(1933). As an index of this factor he com- 
puted the ratio of standard deviation to mean 
annual rainfall for each of certain stations. 
Isopleths of this index showed that the smallest 
year-to-year variation occurred in the high rain- 
fall zones of the mountains and the largest 
variations in the leeward, drier areas. 
Landsberg (MS.) found that for 11 Oahu 
stations, every month of the year had had, in 
one individual year or another, the smallest 
monthly rainfall total in the year. As a measure 
of the skewness or asymmetry of the monthly 
rainfall frequency distributions he computed the 
median-mean . t . , , 
ror each or the 12 months, 
value 
mean 
at a given station. By averaging the ratios for 
the 12 months, each station would be repre- 
sented by an index of asymmetry. These values 
for each of 22 Oahu stations plotted on a map 
showed that the greatest asymmetry occurred in 
lower leeward stations, and the least in windward 
and mountain stations. This is similar to the 
coefficient of variation computed by Nakamura 
and described above, and it checks his geo- 
graphic distribution. 
Another indication of the smaller variations 
in rainfall at higher stations is provided by 
Wentworth (1946), who studied the inter- 
station correlation of amounts of annual rain- 
fall on Oahu. Annual rainfall values at various 
pairs of stations were expressed as ratios, year 
by year. Wentworth computed the mean ratio 
between pairs of stations as part of an analysis 
of the frequency distribution of these ratios. 
