OCR Text |
Show abundant fall of this type of snow, when in half a day a layer 10 mm thick, of density 0.06, was formed. Whenever the opinion is expressed that the snow cover in moderate latitudes often consists of " powdery" snow, a soft snow in the first stage of crystallization is evidently meant. Moist Snow The crystals of moist snow carry no water droplets visible to the naked eye. ( Type) A: Carrying little moisture and loosely bound, this moist snow consists of small needles and columns and does not cover tracks. This type of snow cannot be formed into snowballs immediately following a fall; it just falls apart. The capacity to cling is acquired only after several hours following its deposition. ( Type) B: This type of moist snow is made up of medium- sized stars and platelets and often falls in flakes, sticking to objects and forming bulges in fantastic shapes. It takes a clearly defined print, becomes slippery when trampled, and is easily formed into snowballs. When laterally compressed, this kind of snow does not give way, but forms folds and bumps ahead of the compressing force. It falls at temperatures of from - 5" to + 2° C and is more frequent in the fall and spring than any other type of snow, often building up a considerable accumulation. If this snow falls when the wind is blowing, however slightly, the cover surface is covered with small ripples. At freezing temperatures this snow sticks ( clings) in a porous, unstable, fragile mass of density 0.08 - 0.15. It is not transported by wind, and no case is known of an avalanche following a fresh snowfall, a circumstance which can be explained by the fact that this type of snow never lays down a heavy layer. A layer of soft snow which even partly preserves its structure is transformed by thaws into the type above. In such case its density can increase to 0.25; and if a considerable layer is formed, an avalanche can result. Wet Snow Wet snow consists of crystals which have lost their original shape by thawing and which always carry water droplets. Wet snow often falls in flakes, sometimes in association with rain, at plus or minus temperatures, but always around 0° C. It produces mud on the ground and forms an ice crust when fresh. It has a density of 0.20 and more, and is never transported by wind. During thaws or rains fresh fine- grained snow is transformed into wet snow. If a thick enough layer of such snow is formed, avalanches can follow. Snow Transfer by Wind A wind with a velocity of 4- 5 m/ sec. can transfer snow. Here the wind velocity is 2 m/ sec. at 20 cm above ground surface. Snow crystals roll over the snow surface and hardly rise above it. This movement is hardly visible, the snow surface seeming to be covered by a light fog. While in motion, the snow crystals break up into snow granules and dust. Filling the pores and cracks on the snow cover, these granules and dust further bind the surface, producing a cover sheet that is not nearly so easily broken as fresh snow. The layer is - 4- |