Weather Modification by Cloud Seeding — A Status Report 1989–1997

Abstract

In this chapter the results and status of weather modification research during the period 1989-1997 are examined. I focus on three methods of cloud seeding: the “static mode” and “dynamic mode” of seeding supercooled clouds, and the hygroscopic seeding of warm clouds.

During this period the scientific basis of static seeding has undergone much scrutiny and evaluation resulting in considerable controversy in the literature. While some of the recent work bolsters the early optimism of cloud seeding, overall the image of the scientific credibility of the static seeding concept has been tarnished more than it has been enhanced. One fallout is that scientific research in weather modification has been essentially curtailed in all the developed countries.

While the dynamic seeding concept offers the potential for greater rainfall enhancements than static seeding, it is a much more complex concept. The basic working hypothesis behind static seeding has undergone substantial revision during the period. There is increasing evidence that dynamic seeding can increase rainfall from radar-defined floating targets, but demonstrations of consistent increases in rainfall over fixed ground targets remains elusive. It still remains an unproven candidate for application to water resource management.

While the optimism for enhancing precipitation by glaciogenic seeding of supercooled clouds has waned, optimism for the potential of hygroscopic seedirrg has grown. This is a result of the refinement of hygroscopic seeding delivery systems, modeling studies, and the results of observations from purposefully- and inadvertently-modified clouds. Nonetheless this work is still very exploratory and is a long way from proving that hygroscopic seeding can result in reliable, significant increases in rainfall on the ground.

We conclude by discussing the parallels between purposeful weather modification and studies of anthropogenic influences on global climate and consider:

• The importance of natural variability,

• The dangers of overselling,

• The capricious administration of science, and

• scientific credibility and advocacy.