OCR Text |
Show for a 24- hour period increased to 0.4- 0.5 mm as against 0.1- 0.2 mm for the preceding one and a half months. Insofar as this type of sudden sinking is nearly instantaneous, with collapse of a layer, it may be termed a cave- in. This may sometimes be caused in a field or plot of ground by a pedestrian or skier, and often only a few days are needed to make a snowcover that could support a man cave in under one's feet. These cave- ins may be observed either in valleys or on mountain sides. The extent of the collapse area depends upon the thickness of the collapsed layer and on the firmness of the collapsed layer and on the firmness of the settling snow. When the underlying layer is not very thick and the superimposed layers are not very strong, the cave- in will not involve a large area. But when the collapsed layer is thick enough, not less than 10- 15 cm, and the top layer is firm and strong enough not to crumble readily, the collapse will extend over an area tens of meters on a side and will be accompanied by a characteristic noise like distant thunder. Although the snow gives vertically but a few millimeters, the collapse can be distinctly felt by a man standing on it. These cave- ins are observed during the second part of winter; in all cases checked by digging observation holes, a collapsed layer of granular or coarse granular snow, most often fibrous in structure, is discovered. Due to recrystallization the strength of this layer has decreased to the point where it will no longer support the overlying layers. Collapse can also occur when windblown snow forms a firm layer over a particularly thick layer of fresh or recently fallen snow. Here, when the top firm layer rests on stones or other high relief points, the fresh snow below sinks rapidly, leaving an empty space. The moment arrives when the hanging overlayer collapses of its own weight, breaking up into ragged plates. External causes such as snowfalls, windstorms, the weight of skier, etc., may also result in these cave- ins. Snowcover Temperature Snow temperatures were taken every 5- 10 cm in the test holes at the same time observations were being made of changes in the snowcover. The thermometer, a mercury adaption of a large model Assman- type aspiration psychrometer, was inserted horizontally into the snow at 5- 10 cm intervals. Since direct solar radiation influences thermometer temperatures to some depth, we shaded the test snow during clear weather. The following examples illustrate the effects of solar radiation of temperature in snow. Height of sun above horizon: 20- 21°; Table 3 Snow Temperature in Sun and Shade ( C° ) Distance From Snow Surface In Shade In Sun Surface - 4.0 10 cm - 5.1 - 4.2 20 cm - 4.7 - 4.3 30 cm - 3.9 - 3.9 - 25- |