OCR Text |
Show A h- yea. r field program was undertaken using a dense precipitation- gage network composed of l68 gages in an area of 20,000 square miles or 0.8U gages per 100 square miles, ( The average precipitation gage density in the United States is 0.3 gage per 100 square miles.) The program provided statistical evidence that precipitation was doubled ( from about 0.1 inch not seeded to 0.2 inch seeded during a 2k- hour period) over an area 70 or more miles downwind from the primary project area. The observed results would occur by chance only once in 20 times. The increase apparently results from extending the duration of precipitation rather than increasing the intensity. The reason for the increase is obscure, but there is reason to believe that both microphysical and dynamic factors are involved, although micro- physical factors appear to be of primary importance. Although a downwind effect has been detected statistically when silver iodide has been used to seed winter orographic storms, the effect is obscure when silver iodide is used to seed summer cumulus clouds. Some investigators have reported weak evidence of a downwind effect, while others have found no indication of this effect. Neither of these has been statistically significant. Winter orographic storms cover several thousand square miles. Summer storms cover a few hundred square miles at most. The winter storms have continuous clouds, while summer storms are isolated cloud masses. If the inference that microphysical process is the dominant factor is correct, then the process can readily propagate downwind through a winter storm, but the isolated summer storms preclude propagation. Seeding cumulus clouds with hygroscopic materials produces no downwind effect. These materials affect only the coalescence process and cause no dynamic effects. Avalanches Studies of the problem of avalanche initiation demonstrate the complex interrelationships among terrain, climate, and snow stratigraphy. The Bureau of Reclamation has started an investigation into 514 |