The Effect of Persistence of AgI on Randomized Weather Modification Experiments

Abstract

The hypothesis was tested that AgI may persist for long periods of time and contaminate control experimental days when these control days immediately follow a seeded day. When the first 6 h periods of all such potentially contaminated days were placed in the seeded population for two randomized weather modification experiments, the Colorado River Basin Pilot Project (CRBPP) in the San Juan Mountains and the Jemez Atmospheric Water Resources Research Project (JAWRRP), a statistically significant seeding effect was detected. When the first 6 h periods were not included in either population a significant seeding effect was also detected. However, when the possibly contaminated periods were included in the control population, an inconclusive seeding effect was noted. Support for the hypothesis was obtained by making aircraft measurements of ice nuclei on days following seeded days in the San Juan experiment. Ice nuclei counts of 2 to 3 orders of magnitude above background were found under certain conditions.