Academy of Sciences and a Special Commission of the National Science 

 Board. Their reports, issued early in 1966, were moderately optimistic. The 

 conclusions of the 1953 Advisory Committee that the order of a 10-per- 

 cent increase in precipitation can be expected from seeding orographic 

 storms in western United States were substantiated. Subsequent studies by 

 the Academy and the Interdepartmental Committee for Atmospheric Sci- 

 ences have reinforced early findings. 



PRESENT STATE OF THE ART 



• For certain meteorological conditions the evidence is persuasive that 

 it is possible to increase precipitation by substantial amounts and on 

 other occasions to decrease precipitation by substantial amounts. 



• There is ambiguous evidence that the effects of seeding may influence 

 precipitation at points 100 to 200 kilometers from the site of the 

 seeding. This matter must be clarified. 



• It now appears possible to acquire the additional knowledge neces- 

 sary to predict the effects of seeding on a wide variety of cloud types 

 and systems (convective, orographic, stratiform, migratory storm sys- 

 tems, etc.) in different geographic areas from reasonably realistic 

 computerized cloud models. 



• Supercooled fog can be dissipated on an operational basis. 



• There is encouraging evidence that hail can be suppressed. 



• There is encouraging evidence that the intensity of winds in a hurri- 

 cane can be redilced. 



• There is evidence that further development will lead to operational 

 techniques for decreasing the frequency and duration of cloud-to- 

 ground lightning discharges, with a subsequent reduction in forest 

 fires. 



• Advances in remote-sensing techniques are the first steps toward meth- 

 ods to modify tornadoes. 



• No completely accepted technique yet exists for dissipating warm 

 fog, but the potential economic benefits and the encouraging prospects 

 of such a capability warrant further research. 



• The prospects of inadvertent modification of weather and climate by 

 changing the chemical composition of the atmosphere, the particle 

 concentration, or by the discharge of heat are so real, and so likely 

 to be realized within a matter of decades, that a major program of 

 research appears to be warranted. 



• Weather modification issues now reach to the stratosphere. It has been 

 suggested that exhaust emissions from SST's may decrease the ozone 

 concentration at high altitude and lead to an increase in ultraviolet 

 radiation at the Earth's surface. Fortunately, the way appears clear 

 to resolve this question before SST's are operational. 



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