OCR Text |
Show This report sets forth, mainly in graphic form, the ceasity values of 2ít- hour snowfalls as recorded at several avalanche study cites in the conter*- ndnous United States and Alaska. Investigation of the reccrds was originalliy initiated to answer the question of just what constitutes a " normal" or " average" new snow density in a given área. This normal valae has some bear>- ing on avalanche forecasting, for it has been clearly estailished that avalamche hazard is often associated with snowfalls whose densities depart widely froac the normal. The point of origin for this departure has uesally been taken « ± a new snow density around 0.10 gm/ cc, but the actual mean value in a given climatic zone may differ from this figure. The new snow densities referred to hereafter in this report are to be cnder-stood as 2if- hour new snow densities- that is, they refer to the density of snow which has accumulated over a period of Zh hours on a snow stake platform rest-ing on the snow surface, and whose depth and water content have been measur- ed early in the morning. Such an observation is a part of the routine snow recorda collected at each of the avalanche stations listed. See Department of Agri-culture Handbook No. 19* f, " Snow Avalanches", for details of observation methods. Similar snow records are collected at each U. S. Weather Bureau Climat-ological Station, but very few of these stations are located at high altitudes in the mountains, and practically none at the altitudes of avalanche fracture zones. The data in this report have therefore been drawn only from the avalanche stations, where the location is 6uitable and the observations have been made to. a common standard. Results have been plotted as bar graphs showing the frequency of occur-rence of each new snow density at each station analyzed. These plots are displayed in Figures 1 through 5- The data are summarized in Table I, together with lengths of records and station altitudes. |