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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- 39 -

reinvestigated the calibration of the optical snow-rate sensor for a wide
variety of snow cr'Ystal types \oo"hich occur in the Casc::l.de Mountains. The
results are shown in Figs. 2.2 and 2.3. It can be se.:m that the reading of

the optical snow-rate sensor is linearly related to the precipitation rate

onlv if the snow crystals are unrim€d (Fig. 2.2), in which case our

obscp;.rations give

r = 0.36 d

(2.2)

As the amount of riming on the snow particles increases the relationship

between rand d becomes more randc~ until with g~aupel particles it is

completely random. The actual habit of the crystal docs not appear to affect

the relationship between rand d (Fig. 2.3).

In view of the above observations it is clear that considerable caution

must be used in interpreting the outputs from optical snow-rate ser.sors of the

type descri0ed above in terms of a precipitation rate. However, they can be

used as snow crystal counters. Also, if there is no wind blowing, the number

of s~owflakes which interrupt the light beam in a given period of time can be

used in conjunction with a simultaneous measurement of the precipitation

rate to give the average mass of the falling snow crystals. The average

masses of snow crystals which fall in the Cascade Mountains, which we have

deduced by using this technique, range from I to 2 milligrams. This is one

to three orders of magnitude greater than the masses calculated from the

size-mass relationships for ice crystals given by Nakaya and Terada (1934).

The difference may be due to the large aggregates which are common in the

Cascades.