300 
minutes would be required for a 30-micron di- 
ameter drop to grow (by accretion and coa- 
lescence under the same conditions) from this 
size to a 100-micron diameter.) It can be seen 
that there is a considerable discrepancy between 
these computations and the period of 3 min or 
so elapsing from the time a radar echo overhead 
is detected by a sensitive radar and 3-mm drops 
are collected at the mountaim summit. 
A further discrepancy is that initial echoes 
were observed several times in vigorous clouds 
20 min or less after the clouds first formed and 
rain fell from these clouds a few minutes later. 
From the appearance of these echoes it would 
seem that they formed by coalescence mecha- 
nisms, since a bright cup echo was observed, with 
no change in intensity at the melting level and 
some of these echoes first appeared below the 
level of the 0°C isotherm. Application of col- 
lection-efficiency computations based on aero- 
dynamic and geometric assumptions would indi- 
cate that the drops need a much longer period 
from the time the cloud is formed to grow by 
coalescence to the size at which radar echoes 
might be detected (possibly more than 100 min) 
or to fall from the cloud as rain (possibly 30 
min more). 
Our observations of appreciable electric fields 
within the clouds during this period of rapid 
drop growth suggest that electrical enhancement 
of coalescence may actually oecur. The disap- 
pearance of the electrification within the cloud 
as the rain developed and as vigorous convection 
was observed to cease makes it difficult to sup- 
port an argument that the precipitation causes 
the electrification. The characteristic cup shape 
of many initial echoes [Vonnegut and others, 
1958] may be a significant connection between 
the coalescence and the distribution of electric 
fields within a convective cloud. The cup shape 
may arise from a coalescence process and may 
depict the distribution of a region of high po- 
tential gradient within these clouds. 
CoNCLUSION 
Our observations of the rapid appearance of 
rain from clouds in New Mexico support the 
ideas that electrical effects appreciably accelerate 
the coalescence process. 
We have examined our data for days when the 
first rain formed overhead and find that in the 
more vigorous clouds of New Mexico, rain echoes 
sometimes appear (within electrified clouds) in 
as little as 19 min after the cloud is first formed. 
MOORE AND VONNEGUT 
In these Cumuli the rate of increase of cloud 
reflectivity is quite remarkable; sometimes 3- 
mm-diameter raindrops fall in as little as 8 min 
after an initial echo appears. On computing ef- 
fective collection efficiencies for drops in these 
cases we obtain values several times unity. On 
the other hand, computations with present 
coalescence models of the time required for un- 
electrified drops to grow from 0.1 mm to 1 mm 
diameter suggest that about 30 min would be re- 
quired for such a size change and that two hours 
may be required from the time the cloud appears 
until rain is formed. 
Since both the formation of an echo and the 
arrival of rain (sometimes apparently formed 
by coalescence) from the Cumuli in New Mexico 
occur much too rapidly to be described properly 
by present coalescence ideas, the possible effects 
of the observed cloud electrification must be con- 
sidered. It appears desirable to carry out further 
studies that will provide a detailed picture of the 
precipitation growth process from which more 
defensible collection efficiencies can be com- 
puted. 
Another experimental approach that may shed 
light on the coalescence process is to determine 
whether high precipitation growth rates such 
as we observe ever occur in clouds that have not 
become electrified. The examination of raindrop 
growth rates in warm clouds should be of ex- 
treme interest. 
Acknowledgments—This work was made pos- 
sible by the support of the Office of Naval Re- 
search, Bureau of Aeronautics, The Atomic En- 
ergy Commission and The National Science 
Foundation under Contract Nonr 1684(00). 
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