Diurnal Weather Patterns— Leopold 
87 
In addition to showing the breakdown of 
highly significant 8-hour periods given in 
Table 1, the Honolulu record was sufficiently 
long to show significance in individual hours. 
Figure 3D shows the occurrences hour by hour 
for Honolulu. This provides a somewhat more 
complete picture of the diurnal curve of rain¬ 
fall occurrence for a station receiving predom¬ 
inantly trade-wind orographic or nocturnal rain. 
At appropriate ordinate values a long-dashed 
line shows the number of occurrences necessary 
in an individual hour for high significance. 
That is, if we assume all hours have an equal 
chance for rainfall, there is less than one chance 
in 100 that random drawing would provide an 
individual hour with a larger number of occur¬ 
rences. Similarly, the lower dashed line shows 
the lowest number an individual hour should 
receive in 100 trials of random drawing. 
Comparison of Diurnal Rainfall Curves at 
Various Stations 
It is evident that certain stations have a rain¬ 
fall maximum during the night, which is gen¬ 
erally associated with a minimum in the after¬ 
noon. Stations having nocturnal maxima are 
Honolulu, Kawailoa Girls’ School, Waimea, and 
Opaeula No. 8. The opposite case is a maxi¬ 
mum in the afternoon and a minimum during 
nighttime hours. This group includes No. 537 
Lanai, and Leilehua. Kawaihapai has no sig¬ 
nificant difference between hours. No. 5519 
Lanai shows an afternoon maximum in spring 
and summer. 
The four Oahu gages which lie near the 
Koolau Range, including Kawailoa Girls' 
School on the windward side, show nocturnal 
maxima. The two gages at some distance from 
the Koolau Range-—that is, Leilehua and Kawai¬ 
hapai—showed either no significant difference 
between hours or an afternoon maximum. 
The difference is the dominance of orographic 
rain from the trade winds as against convective 
showers. The latter are an afternoon phenome¬ 
non while trade-wind orographic precipitation 
occurs primarily at night. 
Lanai, as an island, is much drier than Oahu 
and shows other peculiarities due to the fact 
that it is partly in the rain shadow of the much 
higher island of Maui. Both recording gages 
on Lanai have afternoon rainfall maxima. 
Though they are relatively close to the moun¬ 
tains, which are just upwind, apparently the 
overall rainfall deficiency on Lanai due to the 
rain-shadow effect diminishes the importance of 
orographic trade-wind showers. Opaeula No. 
8 lies twice as far from the mountain crest as 
the Lanai gages, yet still shows nocturnal 
maxima. It will be noted, however, that Opaeula 
No. 8 had no highly significant hours of rainfall 
minima. This should probably be interpreted 
as an indication of diminishing importance of 
orographic rains owing to the distance from 
the crest. 
Again, Leilehua lies only 2 miles upwind of 
the crest along the central mass of the Waianae 
Range. The station nevertheless shows pre¬ 
dominantly afternoon rainfall maxima. Kawai¬ 
hapai lies some distance downwind of the lower 
northerly nose of the Koolau Range. It is 2.7 
miles upwind of an 1,800 foot crest of the 
Waianae Range. Lack of highly significant 
periods of rainfall indicates the mixed effect of 
orographic and oceanic rain with convective 
showers. The explanation might lie in the few 
total occurrences of rain, but this was tested by 
increasing the length of record and the results 
still gave no significant hours. 
The diurnal rainfall characteristics of Lei¬ 
lehua and the Lanai gages emphasize the im¬ 
portance of the desiccation of air by the barrier 
over which the air is forced to rise. The eleva¬ 
tion of the band of maximum rainfall on East 
Maui and on the volcanoes of Hawaii clearly 
demonstrates this principle. Yet the dominance 
of afternoon or convective rainfall even at the 
base of the windward slopes of the Waianae 
Range and so close to leeward of the Lanai 
Range is a little surprising. 
The statistical results presented above from 
the limited number of recording rain gages are 
borne out by the experience of many observers. 
