EFFICIENCY OF NATURAL RAIN 
would greatly accelerate the accretion process. 
The gap between the echoes of the upper light 
rain and the heavy rain in Figure 1 indicates 
that the drops from the light rain did not descend 
into the heavy rain cloud. 
Conclusion—The efficiency of cloud in initiat- 
ing rain increases with cloud depth, LWC, and 
duration. At a critical depth of about 5000 ft for 
warm Cumulus, without unfavorable wind shear 
and mixing with the environment, sufficient liquid 
water content is established for cloud drops to 
grow to raindrop size. It is doubtful that clouds 
fail to rain because of the lack of large drops be- 
cause observation indicates that some clouds over 
the land produce rain while some clouds over the 
sea with more numerous larger drops fail to pro- 
duce rain. 
Critical depths and liquid water content for 
precipitation formation via the ice phase are 
smaller than in warm clouds. For the formation 
of drizzle in stratiform clouds critical depths are 
also smaller but critical durations are probably 
greater. 
The efficiency of cloud in utilizing available 
water is greatest in the middle of widespread 
161 
precipitation and least in the individual thunder- 
storm. Rain amounts in Cumulus are greater per 
unit depth of cloud than in stratiform clouds. 
Very efficient from both viewpoints is the case 
of a Cumulus cloud imbedded in a stratiform 
deck. 
The author is indebted to Helmut Weickmann 
for suggesting the topic and for many helpful 
suggestions, and to Edwin Kessler III for use of 
the October 1, 1958 radar data. 
REFERENCES 
Barran, L. J. anp R. R. Branam, A study of con- 
vective precipitation based on cloud and radar 
observations. J. Met., 13, 587-591, 1956. 
Batran, L. J. anp C. H. Rerran, Droplet size meas- 
urements in convective clouds, Artificial Stimula- 
lation of Rain, (Helmut Weickmann and Waldo 
Smith, eds.), Pergamon Press, pp. 184-191, 1957. 
Branam, R. R., The water and energy budgets in 
the thunderstorm. J. Met., 9, 227-242, 1952. 
WeickMaNn, H., The efficiency of natural rain and 
its determination, USASRDL Tech. Rep. 1973, 
14 pp., 1958. 
Wexter, R. anp D. Attias, Moisture supply and 
growth of stratiform precipitation, J. Met., 15, 
531-538, 1958. 
Discussion 
Mr. R. D. Elliott—I feel that in treating any 
precipitation mechanism it is necessary to inte- 
grate over the entire mechanism, and when we 
consider a major storm, it is necessary to con- 
sider more than a vertical column. In extending 
the consideration to the outer boundaries of the 
storm it is necessary, to put two more terms 
into that equation: (1) The horizontal transport 
which may have a relatively small magnitude in 
the column, but when integrated over a storm, 
becomes important; and (2) An evaporation 
term which may be very small in the particular 
column where all the rain is falling, but out 
on the boundaries of the storm and on the for- 
ward edge and possibly the top, can be large. 
It would appear to me that in the studies of 
a large scale precipitation mechanism it would 
be necessary to sample over the entire storm and 
get a complete water budget. 
Dr. R. Wexler—I quite agree with you. There 
is no doubt that the advection term is important. 
In the case treated previously, I was concerned 
with the stratiform case in which one could 
reasonably leave out the horizontal advection 
terms. In a thunderstorm or any precipitation 
from a cumuliform cloud, one cannot. 
Dr. B. J. Mason—I would like to take up a 
point concerning the time and space requirements 
for the growth of a raindrop, which Dr. Wexler 
raised. Clouds over the tropical maritime ocean 
of little more than a kilometer thick may pro- 
duce precipitation. Over England we get coales- 
cence rain from a cloud perhaps five thous- 
and feet deep. In both cases, if one tries to follow 
the history of a raindrop, one finds that the time 
for which the cloud exists is a very important 
parameter. Assuming a uniform updraft, it is 
quite impossible in the space available in the 
cloud and in the time available to get a raindrop 
out. This means one must have a much more 
complicated picture of the vertical motion, and 
one way to do this is by means of successive 
thermals. It may be the last thermal which is 
very important, and indeed my theoretical analy- 
sis shows that this must be the case, if one wants 
to get a shower out of a cloud only one kilometer 
deep. If you take the exact same parameters, 
but a homogeneous updraft speed, nearly two 
