OCR Text |
Show - 6 - routine, and also occasionally at other Forest Service avalanche stations. Diamond and Lowry ( 6) have reported such observations from the Central Sierra Snow Laboratory at Dormer Summit, California, and have related the observed values to temperatures in the upper atmosphere. Attempts to relate Alta new snow densities to upper air temperatures have shown very poor correlation. This is attributed to strong orographic lifting of air masses over the Wasatch Mountains, such that upper air soundings from stations in the Intermountain Basin bear little relation to temperatures prevailing above Alta. The true new snow densities observed at Alta over a number of years do show a definite relationship to the local air temperature as observed in a standard Weather Bureau instrument shelter. This relationship is presented in Figure 6. The data collected so far suggest a general, non- linear increase of new snow density with air temperature. A wide range of densities are observed at air temperatures within a few degrees of the freezing point, but this range becomes more constricted as air temperature falls, until relatively little variation is found below - 10° C. In brief, fairly low densities as well as high ones are observed at warmer temperatures, but high densities are not found at low temperatures. An examination of these true new snow densities in relation to snow crystal types has begun, but as yet data are insufficient to permit drawing any general conclusions. It can be noted that the higher values of new snow density at Alta ( above 0.15 g/ cm3) are usually associated with graupel or needle crystals. The highest true new snow density yet observed at this site is 0,33 g/ cm3, resulting from a snowfall near the freezing point which consisted entirely of needle crystals ( International Snow Classification Type 4). This value is not plotted in Figure 6. |