OCR Text |
Show THE EVIDENCE AND ITS INTERPRETATION Rimed Snow Crystals. The accreation of rime particles ( frozen supercooled water droplets) on falling snow crystals appears to be a common phenomenon. Yosida ( 1955) reports that most of the snow falling at Sapporo, Japan, exhibits rimed crystals. A comparable condition is found at Alta, Utah, USA, where completely rime- free snow crystals may occur only in one snowfall out of ten. There is little information available on the climatology and geography of crystal riming, for observations of this phenomenon is not a part of standard meteorological records. The percentage of occurrence may well be much lower in some localities. The famous collection of snow crystal photographs by Bentleyin Vermont, USA, contains very few examples of rained crystals, although the degree of selection that was exercised has not been stated ( Bentley and Humphreys, 1931). The required presence of a super- cooled water cloud eliminates the possibility o of crystal riming at temperatures below - 40 C. Riming appears to be most common when precipitation at moderate temperatures is generated from pre- fronta clouds by strong orographic lifting. The observed effect ranges through a complete spectrum from a few scattered rime particles per crystal to the dense pellets of graupel. Data Sources. The following observations on the effects of snow crystal riming on avalanche formation have been made over a period of 13 years, largely at Alta, Utah. This site is located in the Wasatch Mountains of Utah, which rise abruptly along the eastern margin of the Great Basin. |