Elevation and Spatial Variation Effects of Wintertime Orographic Cloud Seeding

Abstract

This study is concerned with the elevation and spatial variation effects of wintertime orographic cloud seeding over an area encompassing Fremont, Hoosier and Vail mountain passes in the central Colorado mountains during a period from 1960 to 1965. The observation network consisted of 65 precipitation stations distributed over the three passes. Depending on the grouping of precipitation stations used to represent the prime target area of the study, the average daily precipitation for all 120 seeded days was from 6 to 11% greater than the average daily precipitation for all 131 non-seeded days. There is a high probability that these differences could have occurred by chance alone. Analyses have also been made according to physically defined stratifications based on a model which describes the seeding effects ascribed to the various strata. Statistically significant increases (decreases) were observed over much of the area for the seeded periods in comparison with the non-seeded periods when 500 mb temperatures were ?20C and warmer (?27C and colder). Little or no effects were noted in the intermediate temperature range. When 500-mb wind velocities were from 22?28 m sec?1, statistically significant increases were observed during the seeded period in comparison with the non-seeded period throughout the area.