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PACIFIC SCIENCE, Vol. Ill, July, 1949 
marked by frequent interaction between wave 
troughs and ridges in the polar westerlies and 
tropical perturbations. Especially the larger 
amounts of rainfall over Hawaii seem to occur 
mainly in periods of such interaction, while un- 
complicated passages of extratropical fronts 
account for many of the smaller rains. Inter- 
action between disturbances of high and low 
latitudes apparently is the most potent rain- 
producing factor throughout the trade wind 
belt.” 
In describing cyclonic storms, Jones pictured 
the cloud types producing winter rains as com- 
parable to middle latitude frontal cloud systems. 
This is true infrequently. Hawaii seldom ex- 
periences a complete overcast of altostratus or 
nimbostratus clouds. In a large percentage of 
storm periods, the cloud systems are more like 
those described by Riehl (1945) in the Carib- 
bean. On rainy days over the ocean the masses 
of cumulus are larger and higher than usual, 
built up into congestus and, occasionally, cumu- 
lonimbus clouds. Patches of altostratus are 
usually associated with these cumulus masses, 
with occasional cirrus and altocumulus clouds. 
Over the islands, orographic lifting assists in 
cloud development, and the coalescing cumuli, 
viewed from below, often resemble nimbostra- 
tus. From afar it can be seen, however, that only 
segments of an island are under a complete 
overcast. 
In summary, the rainfall usually comes from 
large but discrete cloud masses mostly of cumuli- 
form type. A rainy day is characterized by in- 
creased size and number of cumuli form clouds. 
These changes result from horizontal converg- 
ence to the east (the rear) of the axis of an 
easterly wave and in association with a trough 
in a westerly current. 
STORM TYPES IN RELATION TO RAINFALL 
Studies of weather types correlated with rain- 
fall amounts in Hawaii have been made by 
Huddleston (MS.) and by Wallen and Yeh 
(MS.). In both cases the authors used the 
Northern Hemisphere Surface Synoptic Chart 
Series. Huddleston used the precipitation re- 
corded at Honolulu and the CIT-AAF 3 classi- 
fication system (1942). He also had available 
the 3 km. maps accompanying the Northern 
Hemisphere surface charts. 
Huddleston showed that the best correla- 
tions with Honolulu rainfall were obtained 
with a type he called "No. 31,” characterized 
by "purely zonal flow at the surface” with a 
series of cyclones moving eastward along the 
Aleutian chain. The 3 km. level is characterized 
by "weak circulation with the base of the upper 
trough extending as far south as the Hawaiian 
Islands . . . .” Though his explanation is not very 
clear, his map gives the impression that the 
situation involves the passage over Hawaii of a 
polar trough of the type described by Riehl 
(1945). Huddlestons second important rain- 
producing type is his No. 22, "Meridional cir- 
culation . . . [with a] Well developed Pacific 
High” positioned over the ocean 20° off the 
Oregon coast, and a "semi-stationary low center” 
northeast of Hawaii at about 30° N., 150° W. 
The abnormally high latitude position of the 
Pacific High is associated with a low index, and 
some frontal systems or troughs may move at 
relatively low latitudes over Johnston Island 
and Hawaii, stagnating in the semi-stationary 
trough northeast of Hawaii. The passage of 
these troughs over Hawaii would account for 
the correlation of the map-type with rainfall. 
Wallen and Yeh (MS.) set up a surface map- 
type classification scheme which expressed the 
geographic relation of frontal systems and high 
pressure cells to Hawaii. The Hawaiian area 
was inspected on the daily Northern Hemis- 
sphere surface charts and the maps were classi- 
fied for a 2 5 -year period. The two main cate- 
gories of their classification scheme were de- 
scribed as "anticyclonic” and "cyclonic.” The 
former placed a high pressure cell NW or NE of 
Hawaii giving surface winds over the islands 
NE or ESE. The cyclonic situations were char- 
acterized primarily by westerly surface winds 
3 California Institute of Technology-Army Air 
Force. 
