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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- 43 -

thermoelectric cooler (see Fig. 4 in Hobbs et al., 1970), cold methanol was

circulated in a closed system. The temperature of the methanol was controlled

by pumping it th~ough a refrigeration unit and then heating it to the desired

temperature. In this way, the temperature of the methanol could be kept

constant to within a few tenths of a degree for-any period of time.

Previously the temperature of the drops was determined by a thermocouple

taped to a polyethylene film. This has now been replaced by a thermistor

imbedded just below the surface of the aluminum plate in which the vacuum

manifold is located. The thermistor is then used to control and observe the

temperature of the plate. The temperature of the drops can now be set at

any value between ~ 400Cwith an accuracy of 0.20C and the temperature

measured to an accuracy of O.loC. The temperature can be increased or
d d f 0 5oC. -1 1. f 20C . -1 d
ecrease at a rate rom to m1n; a coo 1ng rate 0 m1n was use

in all the results described in this report.

During the middle of the 1970-71 program the volume of the individual
3 3
water drops was changed from 0.001 cm to 0.017 cm. This permitted the

concentrations of freezing nuclei to be determined to higher temperatures,

which corresponded more nearly to those of the natural cloud. Fig. 2.4 shows

the measured spectra of freezing nuclei in the same samples of snow using two

different drop sizes. It can be seen that when the volume of a drop was
3
0.017 cm the spectra.could be extended to higher temperatures. However, with

the larger drops the maximum concentrations of freezing nuclei per gram which

could be measured were in the hundreds while with the smaller drops they were

in the thousands.