Conclusion -76- 



Modifying Orographic Thunderstorms. It may be possible that silver- 

 iodide seeding from ground generators would be particularly useful in mod- 

 ifying orographic "towering" cumulus to prevent their growth into thunder- 

 storms. By determining the air trajectory from the ground into the cold 

 part of the cloud, potential ice nuclei may be sent aloft by a very simple 

 procedure. If subsequent experiments indicate that it is important to seed 

 such clouds at a temperature only a few degrees colder than the freezing 

 point, it may become necessary to use dry ice dispensed from planes or 

 carried into the clouds by free balloons or projectiles. 



Modifying Towering Cumulus . Towering cumulus also forms over 

 flat country at times when the atmosphere is conditionally unstable. Dan- 

 gerous and often deadly lightning strokes, torrential rains, destructive 

 winds, and sometimes hail and tornadoes are the end products of such 

 developments. Since the high, vertical thickness of a supercooled cloud 

 seems to be the basic requisite in the formation of a thunderstorm, it may 

 be quite feasible by proper seeding methods to prevent this phase from 

 developing. 



The manner in which the seeding is done may produce a wide vari- 

 ation in the end results obtained. Ey seeding each cumulus tower with 

 large numbers of crystals shortly after it rises above the freezing level, 

 the cloud would be continuously dissipated and no extensive regions of 

 supercooled cloud could develop. On the other hand, it might be desirable 

 to seed such clouds to realize the maximum possible energy release. This 

 presumably would involve seeding each cumulus tower just previous to 

 the point of its maximum development. If this could be done effectively, 

 it might be possible to build the storm into a much larger one than would 

 develop under natural conditions. 



Preventing Hail . The possibility that hailstorms might be prevented 

 by seeding techniques is of considerable economic importance. A great 

 amount of basic information is needed on the various properties of storms 

 that produce hail. In some parts of the country where severe hail damage 

 is frequent, storms are formed over certain mountain ridges and peaks 

 that serve as cloud breakers. Such clouds should be particularly suited 

 for modification by ground generators, since the air trajectory is definitely 

 related to the flow of air up the mountain and into the clouds. 



APPARENT LIMITATIONS 



As in any of the physical phenomena, there are definite limitations 

 to the degree in which experimental meteorology may be employed in mod- 

 ifying clouds in the free atmosphere. Some of these apparent limitations 

 may disappear as our knowledge increases, although most of the restrictions 

 now recognized are imposed by known physical laws. 



