WMOD00595

• Title: Lake Oroville Runoff Enhancement Project - Final Report
• Date: 9/1/1995
• Weather Modification - Doc Type: Report

Figure 5.1b shows yearly statistics from ::\:It. Hough for the 4}'T of record. Mt. Hough is about
300 m (1000 ft) higher than Red Hill, and thus experiences sub-freezing temperatures more
often. This elevation contributes to :\:It. Hough averaging about 500 hours of icing per winter.
The upper histogram on figure 5.1b shows that ::\It. Hough also averages slightly more hours
of icing than hours of precipitation recorded at Plumas Eureka State Park. Average liquid
water contents are low, as was observed at Red Hill. Icing temperature averages just under -
3 "C, or about a degree cooler than Red Hill.

The importance of the results from Red Hill and ::\-It. Hough can be better appreciated by
understanding ice crystal growth rate. Figure 5.2 shows mass growth rate of ice crystals as
a function of temperature and time. This plot only includes ice crystal growth by conversion
of water vapor to ice {diffusional growth), not collection of supercooled droplets by ice
particles (riming). Given the liquid water contents observed during icing events, riming
would only be significant during more limited periods when liquid water is greater than
about 0.1 g m-3. ::\ote that for temperatures warmer than -3 "C, crystal mass even after 30
min is three orders of magnitude lower than a crystal growing at -5 ce. Thus, unless seeded
crystals can attain temperatures near -5 "C, the crystals would require nearly 2 h to equal
the mass of a crystal growing at -5 "'c.

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Figure 5.2. - t,,'asses of growi~g ice crystals at various times as a function of temperature tA'ter E.E. Hindman II and
D.S. Johnson [19721. J. Armes. Sci. 29. '313. by permission of Amer;can t.1eteorological Society and senior author).

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