NOTES 
Hawaii as a Cloud Physics Laboratory 
The major islands of the Hawaiian chain 
lie across the Pacific northeast trade winds 
near north latitude 21°, about 2,400 miles 
southwest of San Francisco. It is the purpose 
of this note to discuss very briefly the great 
advantages of one of these islands (Hawaii) 
as a place to study the relationships between 
the precipitation elements in clear marine air 
and these elements present in clouds formed 
in this air. 
Recent developments in the study of rain 
formation have led to the hypothesis that 
large sea-salt particles are the basic nuclei 
upon which many raindrops initially form 
(Ludlam, 1951, Quart. Jour. Roy. Met. Soc. 
77: 402—417; Woodcock, 1952, Jour. Met. 
9: 200—212; Squires and Woodcock, MS.). 
These developments have re-emphasized the 
need in cloud physics for further detailed 
physical-chemical studies of rain which will 
enable us to ". . . know how nature operates 
to produce precipitation” (Byers, 1953, In- 
dian Acad. Sci., Proc. 37(A): 237—247). 
In order to make preliminary tests of the 
salt particle-raindrop hypothesis by making 
measurements of natural rains, it was neces- 
sary to find a geographic location where the 
contents of the lower atmosphere could be 
determined before, during, and after the for- 
mation of cloud and rain. Recent work has 
indicated that the windward side of the island 
of Hawaii, in the region of Hilo, is almost 
ideal for this study (see Fig, 1). Here, on 
most of the days throughout the year, rain 
falls near area A from orographic clouds which 
have formed over the island and within the 
trade-wind stream. If one follows the stream- 
lines of this trade-wind air upwind of area A 
and over the sea, only scattered cumulus 
clouds are found upon arrival near area B. If 
the air is followed downstream from A, the 
clouds are found to dissipate, leaving clear 
air at c. Hence it is possible to determine, by 
appropriate measurement, the contents of a 
parcel of clear air at B, the nature of the 
precipitation within clouds formed in this air 
when it arrives at A, and the final modified 
parcel after the clouds have dissipated at c, 
a distance of about 30 miles. 
Much of the mountain slope near Hilo is 
readily accessible on paved roadways. Rain 
and cloud drop sizes may be sampled within 
the orographic clouds from the 2,000 foot 
level to the 6,500 foot level on these road- 
ways, thus avoiding the problems of evapora- 
tion, accretion, etc., which can greatly modify 
rains falling through clear air from cloud base 
to ground. 
The presence of the trade-wind inversion at 
an average height of about 6,000 feet over the 
sea concentrates the clouds and water vapor 
in the lower air (Riehl et aL, 1951, Quart. 
Jour. Roy. Met. Soc. 77: 598—626; U.S. 
Weather Bureau, Staff Members, Honolulu, 
MS.; Leopold, 1949, Jour, Met. 6: 312—320). 
The inversion also limits the vertical extent 
of the salt nuclei, as shown in Figure 2. The 
distribution curves representing salt particles 
at the 1,550 and 2,740 meter altitudes show 
a great reduction in the number and weight 
of particles at the base of the inversion and 
above it (see temperature on insert diagram. 
522 
