OCR Text |
Show As the last vestiges of the primary stars vanish, the ice pellets grow and are gradually transformed into grains measuring 2- 3 mm. These irregular ragged grains are transparent and on the whole look like sugar. This type of snow we will call " loose granular snow." Such snow is not firm, but fragile, and crumbles easily. Its density is 0.15- 0.25, and when windblown, as much as 0.35. Grain formation is the second stage of snow recrystallization. In some snow layers, especially the lower ones, recrystallization proceeds further. Transparent grains of regular shapes, quadrilateral and hexagonal plates, rhomboid cubes and prisms, " gutters" ( half a hexagonal prism cut lengthwise), goblets, pyramids with one imperfect side, and fractions of such formations, will appear. The sides of such crystals sometimes have a mirror- like surface, but more often are stepped or escalated. The stepped sides are formed by parallel prisms of equal diameter fusing in the same plane. These prisms grow progressively to enlarge the snow crystal. Occasionally crystals are seen which have not yet grown all the way to the opposite side. Nearly every crystal is found fused with an ice grain of irregular shape, indicating that the grains themselves do not grow, but that large crystals progressively take shape over some of their sides. One often observes fused and doubled crystals. Such snow is comparable to freshly fallen snow in variety of crystal forms. Differences in crystal shape, minimal at the beginning of recrystallization, and which later disappear altogether, reappear in this type of snow. Certain crystal forms prevail in different years at different places. Crystals of the above snow, which may reach proportions of 10 mm and more, are often clumped in fragile, easily disintegrated groups. We shall call this type of snow " loose, coarse, granular snow." Its density is 0.15- 0.30. It represents the third and last stage of recrystallization. In the literature this type of snow is often referred to as " deep hoarfrost" or rime, although the causes of hoarfrost ( radiational cooling of ground and objects) and other sublimational atmospheric phenomena are different. In any given snow layer will be found a considerable amount of particles smaller than the prevalent grains of the same structure, and one sometimes finds coarse crystals in fine- grained snow. When conditions are right, crystal form as well as arrangement undergo change. Snow grains do not fill spaces evenly, but form accumulations elongated vertically as bundles or sheets so that the snow layer acquires a fibrous texture. The first signs of this fibrousness appear in fine- grained snow. Between fibers empty spaces develope which increase in size to several millimeters in granular snow. One can nearly always observe the fibrousness characteristic of coarse- grained snow. The spaces between fibers are such that a pencil can be inserted. Fibrous texture appears in the upper as well as the lower layers of snowcover. The looser the snow, the more fibrous the layer. The appearance and growth of the empty spaces between fibers is due to a redistribution rather than to a reduction in the amount of snow material. In fine granular snow, where the grains do not exceed 0.5 mm, the spaces are measured in fractions of a millimeter. The increase in size of the intergranular spaces parallels the growth in - 9- |