WMOD00317

• Contract/Permit #: 14-06-D-6999
• Project Name: CAWRP
• Title: Cascades Atmosphere Water Resources Program - Volume I
• Prepared For: US Department of the Interior Bureau of Reclaimation
• Date: 7/1/1970
• Weather Modification - Doc Type: Report

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2.3

Crystal Types
At the manned stations plastic replicas were made of the snow crystals
in the air using a 1% solution of Formvar in ethylene dichloride. Glass

slides (2" x 2") were used as a substrate. If the evaporative coolinp; causes

the temperature of the slide to fall below the dewpoint of the air,
condensation occurs on the plastic replicas and this can be mistaken for
natural riming. This problem was overcome by either placing the glass slide
on a thermoelectric cooler surrounded by a supply of dry air or by placing
the slide in a cold box at -20oC immediately after it had been exposed to the
falling snow. Both techniques can also be used to obtain replicas even when
the air temperature is a few degrees above the melting point.
2.4 Snow Sampling

Specially constructed polyethylene bags (60" x 50" in area and 0.00125"

thick) were used to catch large quantities of snow. The bags were stretched

over cylinders (40" in diameter and 24" high) and exposed to the snowfall for

measured periods of time. The bags were then sealed, weighed and stored for

subsequent analysis of the concentrations of freezing nuclei and silver in

the snow as described below.

2.5 Freezing Nucleus Concentrations

The snow samples were used to determine the concentrations of freezing

nuclei in the snow using the drop-freezing technique. The basic theory and

instrumentation used in this technique have been described by Hobbs et al. JL-

(1970), therefore, we mention here only the improvements and changes which have'

been made during the past years. 1-

Instead of using a circulating ice bath as the heat exchanger for the

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