84 DISCUSSION 
concerned; that is, is more or less cloud water 
realized at the ground when precipitation is ini- 
tiated low in the cloud before or simultaneously 
with that in the upper regions. 
Acknowledgment—This research was spon- 
sored primarily by the Institute of Atmospheric 
Physics, University of Arizona, and by the Geo- 
physics Research Directorate under Contract 
No. AF19(604)-1134. 
REFERENCES 
ACKERMAN, Bernice, Characteristics of summer ra- 
dar echoes in Arizona, 1956, Sci. Rep. 11, Inst. 
Atmos. Phys., Univ. of Ariz., Tucson, 1959. 
Bartan, L. J., Observations on the formation and 
spread of precipitation in convective clouds, 
J. Met., 10, 311-324, 1953. 
Workman, E. J., anp 8. E. Reynoups, Electrical 
activity as related to thunderstorm cell growth, 
Bul. Amer. Met. Soc., 30, 142-144, 1949. 
Discussion 
Dr. B. J. Mason—I think Miss Ackerman 
would agree with me that it is difficult to disen- 
tangle the two processes which may be occurring 
in the case where the first echo is both above 
and below the freezing level. I have made ob- 
servations of it, and this brings me to the ques- 
tion, What do we mean by a first echo? A first 
echo is when you first see something on the 
radar screen, but what it means in terms of size 
and so forth of the particles in the cloud depends 
upon the sensitivity and range of the radar. So, 
I would like to ask Miss Ackerman if she could 
give some kind of figure in terms of the sensi- 
tivity of the radar or whatever it might be that 
she takes for a first echo. Experience in England 
is that the first radar echo may just straddle the 
freezing level; but the echoes grow explosively 
in a minute or two both above and below. If you 
catch it a minute too late, you might come to a 
different conclusion, because they change so 
quickly. 
To come to the last point, the day-to-day vari- 
ation, nuclei are the first things microphysicists 
think about but I am inclined to think that fac- 
tors controlling the cloud dynamics may be more 
important. I wonder whether you had at the 
same time made any measurements of the actual 
humidity distributions in the air? 
Miss Bernice Ackerman—The sensitivity of 
the radar was monitored and kept constant so 
we could compare the data. I made rough ealcu- 
lations of the minimum particle size detectable 
by the radar. The calculations give a diameter 
of around 400 microns for a range of 20 mi and 
an assumed water content of 1 g/m*. Of course, 
the size varies with range and droplet concentra- 
tion. 
There was a three-minute time interval be- 
tween two pictures. This could be part of the 
reason for the wide range of temperature (40°C) 
over which these first echoes appeared. I agree 
some of these echoes with low bases could have 
started Just below the freezing level, but this 
still does not mean that they did not develop by 
a water mechanism, and I am not trying to 
separate the two types. In the lower portions of 
these echoes, at least, and possibly in some of 
the upper portions, the growth of droplets is 
through a water mechanism. 
Regarding Dr. Mason’s question on the humid- 
ity distributions, the only type of data I have 
for these days is that available from the regular 
radiosonde. This is, of course, one of the things 
one looks at immediately. There was essentially 
no difference in the vertical lapse of temperature 
and humidity between the days. 
Mr. C. J. Todd—Did you have available any 
visual observation of the cloud growth rate? 
Miss Ackerman—No. 
Dr. W. Hitschfeld—Did you have any infor- 
mation about cloud densities at the time the 
first echo broke out; that is, the amount of wa- 
ter, the quantity of water per cubic meter? 
Miss Ackerman—No, the only data available 
were the radar data. 
Dr. Hitschfeld—I think that cloud-density 
studies, even if calculated only roughly from the 
tephigrams, might allow one to eliminate the un- 
certainty with respect to the precipitation proc- 
ess. If the cloud densities are high enough, it 
might suggest that the East process has a better 
chance of being active (T.W.R. East, Precipita- 
tion of convective water clouds, in Artificial 
Stimulation of Rain, Proceedings of First Woods 
Hole Conference, Pergamon Press, 1957, p. 192— 
201). 
Dr. Donald M. Swingle—Did you make any 
correction for the radar beam width uncertainty ? 
Miss Ackerman—Not as far as the actual cited 
levels are concerned. I am not trying to say 
