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The Cloud Seeding Temperature Window

Lewis O. GrantDept. of Atmospheric Sciences, Colorado State University, Fort Collins 80521

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Robert E. ElliottNorth American Weather Consultants, Goleta, Calif. 93017

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Abstract

The greatest potential for seeding with artificial ice nuclei to augment precipitation should occur with cloud summit temperatures in the range from about −10C to about −25C. This is the temperature region where there may be a deficiency of natural ice-forming nuclei. This cloud-top temperature range therefore constitutes a “temperature window” for seeding effectiveness. This article considers the results from a number of cloud seeding experiments reported in the literature with respect to this temperature window. The analysis of seven randomized experiments and references to four other experiments indicates that there is a window in the cloud-top temperature range for which precipitation increases are indicated. This extends from about −10C to about −24C for seeding conducted in the modes employed on these projects. At the coldest cloud-top temperatures, generally less than about −30C, decreases in precipitation are indicated. There are variations among the samples which appear to be explainable in terms of differences in the degree of convection present, the seeding methods used, or in the type of nucleant employed. No evidence is presented to show that the temperature window concept applies where there are strong dynamic effects, either natural or due to seeding, such as those in relatively large and isolated cumuli.

Abstract

The greatest potential for seeding with artificial ice nuclei to augment precipitation should occur with cloud summit temperatures in the range from about −10C to about −25C. This is the temperature region where there may be a deficiency of natural ice-forming nuclei. This cloud-top temperature range therefore constitutes a “temperature window” for seeding effectiveness. This article considers the results from a number of cloud seeding experiments reported in the literature with respect to this temperature window. The analysis of seven randomized experiments and references to four other experiments indicates that there is a window in the cloud-top temperature range for which precipitation increases are indicated. This extends from about −10C to about −24C for seeding conducted in the modes employed on these projects. At the coldest cloud-top temperatures, generally less than about −30C, decreases in precipitation are indicated. There are variations among the samples which appear to be explainable in terms of differences in the degree of convection present, the seeding methods used, or in the type of nucleant employed. No evidence is presented to show that the temperature window concept applies where there are strong dynamic effects, either natural or due to seeding, such as those in relatively large and isolated cumuli.

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