WMOD00276

• Contract/Permit #: 85-5071
• Title: Cooperative Weather Modification Research Program - 1985 Field Operations Plan
• Date: 1/1/1985
• State: UT
• Weather Modification - Doc Type: Report

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1) cloud top temperatures are colder than -10C and warmer than
-30C,
2) the -SC height is low enough to ensure silver iodide activation
as an ice nucleus upwind of the mountain crest, and cloud base
height is at or below the mountain crest, and

3) wind speeds normal to the mountain range are such that addi-

tional ice crystals produced through cloud seeding will not

blow beyond the target area and evaporate.

According to Swart and Griffith (1983) the first criterion excludes

those clouds with tops warmel. than -10C because such clouds are believed

to be either too warm for efficient production of additional ice

crystals by seeding or elSE! too shallow for sweep-out of significant

amounts of supercooled water by the additional ice crystals. Clouds

with tops colder than -30C are excluded because it is believed that

nucleation, ice crystal production, and precipitation (snowfall) are

already naturally efficient.

The second cdterion excludes those clouds with bases above the

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crest level or very warm storms.

In these cases ice nuclei released

from ground-based generators would not likely be mixed high enough to

reach the -5C level and thus would be ineffective.

The third criterion excludes those clouds in which the additional

ice crystals produced by seeding would be carried downwind beyond the

targeted mountain slopes and evaporate. In the Utah operational project

redistribution of precipitation to the downwind slopes is acceptable,

and the only time this criterion is important is when there may be a net

loss of precipitation due to evaporation. This could happen with cloud

bases above crest level.

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