WMOD00401

• Contract/Permit #: 2-07-81-V0256
• Title: Precipitation Development in Seeded Clouds Over the Sierra Nevada
• Date: 8/1/1986
• Weather Modification - Doc Type: Report

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The sensitivity of aggregation to temperature and vapor
pressure from experimental data was demonstrated by Hosler et
ale (1957) and others who showed that aggregation increased to a
maximum as oOe was approached. At vapor pressures below ice
saturation, no aggregat.ion occurred at temperatures < _40e and
aggregation increased l:apidly as aOe was approached. Under
condi tions of supersaturation wi th respect to ice, aggregation

occurred at all temperatures. An existence of a liquid film was

hypothes i zed to be the real son for these resul ts.

Hosler and Hallgren (1960) observed the growth of small

aggregates between -6 and -25oe in a cloud chamber. A small ice

sphere was suspended on the end of a fiber pointing into the

airstream while small cloud particles were drawn past it at speeds

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approximating the terminal velocity.

They found that the density

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of aggregates formed increased with increasing temperature. The

rate at which aggrega.tes collected other ice particles was

temperature dependent with a maximum at -lloe. At this temperature

the crystal type change:! from a dominance of hexagonal plates to a

mixture of plates and columns and finally to a predominance of

columns at higher tempe:ratures. The collection efficiency of the

crystals indicated a maxima at -lloe, decreasing toward higher

temperatures. The crystal type between -13 to _l7oe (dendritic

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regime) in the experiment was not known.

The change in the

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crystal habit was attributed to the rate of formation of

aggregates; the plates with a larger area of contact, increased

the likelihood of bondinq between two surfaces.

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