DISCUSSION 423 
repeated flights at 20,000 ft altitude: at that al- 
titude, you do not seem to be even halfway to the 
tops of the mature clouds. 
As to the mechanism, of course the latent heat 
released by freezing was suggested very early, but 
I think this may be only a small part of the 
story and that the more important effect is the 
removal from the precipitating cloud of the con- 
densed water that would otherwise cool the air 
off again by re-evaporating in drier air entrained 
into the cloud. You might say that precipitation 
removes latent cold from a cloud and leaves the 
heat locked in. I think it is an interesting and 
fruitful field for investigation to study other 
possible feedback mechanisms. For instance, a 
cloud, once slightly favored on account of latent 
heat, will then also have a competitive advantage 
in utilizing the energy of shear. Also, the down- 
draft chimneys set up by precipitation shafts 
may be important in increasing the efficiency 
of convective overturning. 
The behavior of the clouds strongly suggests 
that feedback of some sort is working here, and 
I do not believe that Dr. Weickmann’s plumbing 
model of precipitation stimulation can be con- 
sidered complete without a valve controlling the 
supply to the spigot that can be operated by 
the water coming out the holes. 
