692 
at a slight angle and therefore having a spiral appear- 
ance, also occur in hurricanes and typhoons. Is there 
some predisposition for extreme convection in the at- 
mosphere to occur in lines? If so, why? 
At this writing, computations of the water budget 
and the energy budget reveal problems of a funda- 
mental nature. One finding is that under average con- 
ditions only 10 per cent of the total water involved in a 
thunderstorm reaches the ground as rain. This is not 
only of significance to hydrologic studies but also places 
limitations on computations of the amount of precipita- 
tion producible by artificial nucleation. Computations 
of the energy budget of the thunderstorm show that 
ideas about thermal instability and its prognostic value 
not only need to be revised but may have to be aban- 
doned altogether. 
Two factors having no very direct connection with 
the thermal stability of the atmosphere appear to be 
important for the formation of thunderstorms: (1) the 
occurrence of local regions of convergence, often too 
small in area to be detected by the existing network of 
upper-wind stations, and (2) the absence of low mois- 
ture in the vicinity of the 315K potential-temperature 
surface. Why other potential-temperature surfaces are 
not very critical in American summer conditions is not 
understood. Perhaps the question is related to the fact 
that 315K is about the potential temperature of the 
dry-adiabatic midafternoon lower troposphere over the 
western plateaus. The importance of the Mexico-Ari- 
zona-New Mexico moist tongue is well known. It should 
again be pointed out, also, that convergence and high- 
level moisture are related. 
The studies point to the desirability of undertaking 
further investigations of isentropic or similar charts and 
of using a net of upper-wind stations with a 50-mile 
or, at most, 100-mile mesh to obtain usable divergence- 
convergence charts. 
Finally, one is not sure why thunderstorms should 
ever occur. The atmosphere seems to be able to take 
care of the vertical heat exchange without such violent 
manifestations. Since thermal instability seems to be 
a necessary but not a sufficient condition for thunder- 
storms, ordinary cumulus convection appears to be 
capable of taking care of the situation. As the thermal 
instability imcreases, why can’t the necessary over- 
turning be accomplished by a great number of fast- 
circulatmg cumulus of small diameter rather than a 
few big thunderstorms which never seem to cover more 
than about 50 per cent of an area of about 55,000 
square miles such as shown on radar at ranges between 
20 and 50 miles? The occurrence of convergent wind 
flow and forced ascent seems to be the answer. 
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