160 
and Atlas, 1958] that storage in the form of 
cloud represents less than 5% of the amount 
of water produced by the updraft, and that the 
precipitation efficiency is about 95% (neglecting 
evaporation from the upper surface of the cloud, 
which should be small). For hght precipitation 
the entire cloud above about —5°C is a re- 
leaser cloud (no liquid water or icing), although 
for heavy rain, storage of cloud liquid water may 
extend to much higher levels. 
In the opposite extreme is the thunderstorm 
in which the precipitation efficiency was found to 
be about 20% [Braham, 1952]; 45% of the avail- 
able moisture was evaporated in downdrafts and 
35% evaporated from the cloud sides or remained 
in the cloud after the cessation of rain. From 
this analysis it 1s evident that most of the loss 
of water is dynamic so that, once the precipita- 
tion has begun, little increase can be obtained 
by increased particle concentration; the greatest 
opportunity for artificial rain production is in 
the initial stages of the cloud when storage of 
cloud liquid water is high. 
From the point of view of the efficiency of rain 
production per unit depth of cloud, stratiform 
280° 
HI GAIN 
1033 E 
H! GAIN 
1035 E 
RAYMOND WEXLER 
clouds are relatively inefficient since a cloud 
deck some 20,000 ft thick gives a rain of a few 
mm per hour, while much higher rainfall rates 
come from Cumulus of smaller depths. Most effi- 
cient from this viewpoint is the case of a Cumu- 
lus cloud imbedded in a stratiform deck. An 
outstanding example is the frontal precipitation 
of October 1, 1958. Radar photographs showed 
that the widespread light rai on both sides of 
the front originated as snow with echo top near 
24,000 ft. The heavy rain with a peak of about 
one inch per hour in a narrow region along the 
front had an echo top at about 10,000 ft (Fig. 1). 
Since the bright band was at 12,000 ft the growth 
of the heavy rain occurred entirely from the 
water phase. The occurrence of heavy rain over 
land from a convective cloud mass of less than 
10,000 ft thick is unusual. In addition to the 
smaller loss of available liquid water by evapora- 
tion, it is reasonable to suppose that drops at 
least the size of drizzle from the surrounding light 
rain were carried into the base of the cloud by the 
strong updrafts, which from the rainfall amounts 
were calculated to be about 2 m sec™* over the 
extent of the cloud. The presence of such drops 
280° 
LO GAIN 
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310" 
LO GAIN 
1035 E 
Fic. 1—RHI’s Blue Hill CPS-9 radar, October 1, 1958; the frontal rain is at a distance of about 
5 miles 
