SNOW, the water vapour in the air crystallized into geo metrical forms. Snow forms usually within, below or between cloud strata, and at various heights within the atmosphere, according to the latitude and the temperature prevailing within the clouds. Snow forms in great quantities within lofty clouds in all latitudes. In the tropics, how ever, it melts into raindrops as it falls, except when it alights upon high mountains. Although most common to the polar region, it doubtless forms in even greater quantity within the north temperate regions, because the air contains much more moisture at lower latitudes than at the poles.
Over the polar regions and upon the tops of lofty mountains perpetual snow covers most of the land to a depth in most cases of many feet. On mountain or plateau regions therein, and on high mountains elsewhere, it ac cumulates to so great an extent and depth that the pressure of the upper masses converts it into ice, thus forming glaciers, which in the polar regions cover thou sands of square miles. In the higher latitudes of the temperate regions it often accumulates to the depth of from one to four or more feet during the winter. It rarely falls in quantity or remains long on the ground on low lands below 3o° of latitude. Owing to many causes, both topographical and meteorological, the amount of snowfall varies markedly upon mountain tops and other locations even when situ ated upon the same parallel degree of latitude. The limit of perpetual snow upon mountains approximates i,000ft. at 5,0ooft. at 6o°; 7,000ft. at 5o° ; io,000ft. at i3,000ft. at 30° ; i5,000ft. at 20° and 17,000f t. at the equator.
The crystalline varieties of snow are for the most part trans parent, and have brilliant facets that reflect light and give them a white appearance in the mass. They vary in size from to to of an inch in diameter, and fall to earth either singly or bunched together into flakes. Flake formation occurs most commonly dur ing the mild, moist snowfalls when the temperature at the ground is 32° or above. Their size depends both upon the temperature wherein they form and the depth of the cloud strata through which they fall or float. Intensely cold clouds are always relatively dry, hence tend to produce the smaller, slow growing and solid type of crystal. Conversely,
warmer clouds (usually the lower clouds) contain much more mois ture and tend to produce the larger, fast growing, branchy type of crystals. Water being a prod uct of the joining of the two gases, hydrogen two parts, oxy gen one part, its nuclear atoms have such an arrangement as to favour the formation of triangu lar or hexagonal forms and to divide into three or six.
The great mass of the crystals of snow have forms whereby they can be roughly grouped into two main classes, columnar and tabular forms. Comprised in the columnar forms are hexagonal columns, both hemihedral and holohedral, and the long, slender, needle-like columns. Grouped in the tabular class are all those, whether solid or branching, that form on a thin, tabular plane. Among the rarer forms are the three or four vaned crystals, twin crystals, four-sided plates and compound forms. The latter con sist of a hexagonal column having solid or branching tabular forms attached at one or both ends, and rarely also at the middle of the column. There are in addition many irregular snow forms, among them icy spicules growing outward at many angles from a granular nucleus, frost-like forms and forms due to twinning, etc. During extreme cold or snowfall from very lofty, cold clouds, tiny columns and solid hexagonal or triangular plates usually predominate.
The order of the types in furnishing the bulk of the snowfall is as follows : branching tabular forms, granular forms, plates with branching exteriors, plate forms, columns, needle forms, com pound forms. The columnar and needle-like forms are, each in their respective classes, much alike, though they vary in size, length, thickness, etc. Compound crystals show much greater variation one from another, owing to the differences in the size, aspect, interior details, etc., of the end plates. Often the bar con necting the end plates is exceedingly short, resembling a tiny thick hexagonal plate. As a result, the end plates grow outward very close together. When one of the end plates is smaller than the opposite one, it converts the crystals into the semblance of tiny parachutes, and they fall with the smaller plate downward.