Review of Persistence Effects of Silver Iodide Cloud Seeding

Abstract

This paper is concerned with persistence effects of cloud seeding for precipitation enhancement.  Effects may last for hours or days.  Persistence of cloud seeding effects means that this environmental technology may affect the microphysical structure of clouds and the development of precipitation for a significant amount of time after the seeding has been completed.  According to Rottner, Brown, and Foehner (1975), persistence may complicate the evaluation of a cloud seeding experiment and reduce the perceived net effect of the seeding.  The sensitivity of the experiment to the actual net effect may be reduced.  Their work in Colorado and New Mexico demonstrated a smaller cloud seeding effect because of persistence.  When no account was taken that cloud seeding material was incorectly present in the conrol period part of the time, the seed/no-seed conrast in precipitation and the effect of seeding were smaller.  When the incorrectly seeded parts of the control period were reassigned to the target period, the contrast and the effect of seeding were greater.  There has been considerable post-analysis of precipitation data associated with cloud seeding experiments in Australia by Bigg and colleagues in a search for persistence effects.  Unfortunately, there apperars to be a flaw in some (but not all) of the analysis which exaggerates the time span (siad to be up to two weeks) of the effects.  Artificial ice nuclei are generated by the cloud seeding aparatus and are injected in various ways into a cloud to increase precipitation.  The surmised connection between higher nucleus concentrations and increased precipitation suggests that measuring and examining ice nucleus concentrations for a period of perhaps a few days after seeding may be worthwhle in a search for persistence.  Bigg and others have suggested that some silver iodide ice nuclei released in cloud seeding may be carried, presumably with precipitation, to the surface.  There the nuclei are believed to stimulate chemical reactions, possibly on plants, that create products that are emitted into the atmosphere to function as a persistent ice nuclei.  An alternative scenario is that the deposited slver iodide modifies or otherwise causes bacteria on the plants to loft into the atmosphere and also act as persistence ice nuclei.