Conclusion -74- 



widespread modifying of weather conditions. Such work could easily have 

 profound economic, political, and military effect. 



Modifying Orographic Clouds . Orographic clouds, which form as 

 moist air is forced to rise when it encounters a barrier such as a mountain 

 range, are very common in mountainous regions, and they often form contin- 

 uously for many days. Relatively little precipitation from them reaches the 

 earth, except as rime deposits on trees and rocks or as scattered snow 

 crystals. If techniques could be devised to cause a widespread and effective 

 precipitation of such clouds, the depth of the snow pack in the vicinity of 

 mountains might be markedly increased. Such a result would be of much 

 importance, since the snow pack on mountain slopes is very valuable in 

 stabilizing the streams which flow from such regions. These streams, in 

 turn, have great significance from a standpoint of electric power and water 

 supply. The work done by the California Electric Power Company (page 70) 

 is an important contribution to this knowledge. 



Producing Regions of Ice Nuclei. The production of specific regions in 

 the free atmosphere containing high concentrations of ice nuclei or potential 

 ice nuclei is an interesting possibility. Cold middle clouds, even though having 

 no appreciable moisture, may be used as "holding reservoirs" to store ice 

 crystals until they come into contact with lower clouds of greater thickness 

 or are entrained into cool or cold cumulus. 



An example of this type of seeding occurred during the hurricane seeding 

 project in October, 1947 (page 61). A relatively thin layer of stratus clouds 

 covering an area of nearly 300 square miles was transformed to snow crystals. 

 The subsequent fate of the crystals is still a moot question, but if a considerable 

 region of them was entrained into the lower levels of a line of towering cumulus 

 observed during the flight and situated in the southeast quandrant of the storm, 

 the entrainment might have exercised a profound effect on the subsequent 

 development of those cumulus clouds. 



Similarly, the ice crystal residue from seeded, but small, cumulus clouds 

 may be entrained at a low level into much larger cumulus forming in their vici- 

 nity. In this way, an effect of considerable magnitude is produced as the super- 

 cooled regions are infected at a lower level than would otherwise be possible. 



It will take much careful study to establish methods for utilizing this 

 type of seeding. Eventually, it may become of great importance. 



Modifying Stratiform Clouds . The widespread modification of stratus 

 clouds by artificial means is possible at the present time whenever such clouds 

 are supercooled. Under such conditions, the clouds may be either further 

 stabilized by overseeding, or precipitation may be triggered by using the op- 

 timum number of ice nuclei. 



