DISCUSSION 
servations of radar echoes and of hail occurrence 
indicate that, at times, hail may reach the 
ground within fifteen minutes of the first ap- 
pearance of echo. In 4 gm m™* Cumulus, coales- 
cence will produce a 50 » radius droplet within 
about ten minutes, at approximately which time 
the first echo will be detected [Hast, 1957]. Fol- 
lowing detection, the droplets may freeze and 
continue growth in the ice phase, or may grow 
further by coalescence before freezing, depend- 
ing upon the temperature. 
Caleulations made (from Fig. 2) of 
the times required for the growth of graupel or 
hail of 1 em diameter, starting with a 50 » ra- 
dius drop, and varying the size at which the 
liquid drop froze. Even considering drops freez- 
ing over a wide range of sizes, and subsequent 
growth over a wide range of particle densities, 
the times to reach 1 em diameter varied only 
from 9 to 16 mins, the shorter time being for 
the low-density growth. These times are com- 
parable to the 15-min interval observed between 
echo detection and appearance, at the ground, 
of the first hail. 
The growth rates and times involved, as dis- 
cussed above, are only applicable until depletion 
of the cloud content becomes significant, and so 
may be relevant only to the first ‘burst’ of hail. 
The shape of the primary graupel nucleus is not 
known, and may not be spherical. It is not 
known whether dense hail begins as graupel or 
as a frozen drop, and it may grow in supercooled 
rain; high density growth by collision with rain 
drops has not been considered, and undoubtedly 
should be. Reliable data on stone structure, and 
on the laboratory growth of various types of 
particles in various environments, are urgently 
needed to clarify these pomts of uncertainty. 
Acknowledgments—The author is on assign- 
ment to the Stormy Weather Group, MeGill 
University, the assistance of whose members is 
gratefully acknowledged. Participation in this 
research, and publication of the above, is with 
the permission of the Director, Meteorological 
Service of Canada. 
This paper constitutes a revision and exten- 
sion of work reported earlier in Stormy Weather 
were 
269 
Group Scientific Report MW-26, Growth of Pre- 
cipitation Elements by Sublimation and Accre- 
tion, May 1957. The present results are consid- 
ered more reliable than in the earlier work. 
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Discussion 
Dr. R. List—Have you taken into account the 
Nakaya-Higuchi observations of drops which 
are caught by the erystals, and which are rolling 
over their surface. I think that this may give a 
greater rate of growth than that by the normal 
sublimation which is taken into account here. 
