OCR Text |
Show Snowfall varies from 9 to 15 meters per year. Winter temperatures are relatively mild but periods of mid- winter thaw are rare. Most snowfall occurs at temperatures between - 5° and - 15° C , although storms at both colder and warmer extremes occur from time to time. The climate lies between the maritime conditions of the Pacific Coast and the continental climate of the high Colorado Rocky Mountains to the east. Character of the snow cover varies widely from year to year, reflecting at one time a continental regime, at another a semi- maritime condition. Large snow avalanches are widespread and frequent, occurring in a wide range of types. It is not easy to evaluate contributory avalanche factors, for many are interrelated. This is especially true for statistical analysis, because adequate data samples collected by reliable observers are hard to obtain. Even if they were available, the significance of a statistical analysis for such complex relations might be difficult to appraise. A rational interpretation of physical cause- and- effeet which is first suggested and later confirmed by field observations is more fruitful for such problems as the interaction between variable weather and ephemeral snow conditions. Such is the approach adopted here. Observations serving as a basis for the present interpretations are of three types: ( 1) Routine records of weather, snow, and avalanches maintained by U. S. Forest Service snow rangers. These records at Alta, Utah, date from 19^ 5; they are complete but vary widely from year to year in quality of technical details. ( 2) A careful study of individual storms, including macrophotography of falling snow crystals, since 1959. These provide excellent technical detail, but |