WMOD00400

• Contract/Permit #: 14-06-D-6801
• Title: Structure and Seedability of San Juan Storms
• Date: 11/1/1976
• Weather Modification - Doc Type: Report

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42

c. Hilliporc filter samples

Hillipore filter samples were also collected and processed in a conven-

tional filter processor. Typical ice nucleus concentrations detected at

water saturation were O.Ol/liter at -lSC, O.OS/liter at -17C, and O.l/liter

at -20C. The highest nucleus concentration obtained on any filter except the

special ones discussed in the next paragraph was 0.3/liter at -20C. We inter-

rret the~e numbers as representing the concentrations of deposition ice

nuclei. Typical sample volumes used were 200-300 liters.

d. Dust as an ice nucleus

On one occasion, we observed unusually high ice nucleus concentrations

in connection 'vith a dust layer that was mixed all the way from the ground

to 3.5 km. This layer contained ice nucleus concentrations, as determined

from filter measurements processed in the deposition mode, of about 2/liter

active at -16C. This is an unusually high ice nucleus concentration, but

this dust probably was still more active in the contact or condensation-

freezing modes. The dust was observed above Farmington in air that was being

blo,Vl1 directly into the storm over the San Juan Mountains. On both flights

of 22 March 1975, the rotating filter measurements indicated unusually high

ice nucleus concentrations below 3.5 km, even well in advance of the seeding

generators. The dust we observed near Farmington may have provided a very

important natural nucleus source. We do not know the frequency with which

such events occur, but such. dust may be associated Hith the strong, well-

organized storms such as the 22 Harch storm. The nucleus concentrations

obtained in the dust layer were the highest we observed in any of the San

Juan flights, including our low passes through seeded storms, and. may compete

seriously with artificial seeding.