SNOW is the water in solution in the at mosphere crystallized into geometrical forms, or congealed by cold into granules or irregular particles. Snow forms usually within the clouds at various heights in the atmosphere, according to the latitude, degree of cold prevailing at a i given locality, altitude of clouds, etc. It produced in great quantity in all latitudes, though it rarely reaches the earth in the tropics (except on high mountain tops), because the heat of the lower air melts it, and converts it into rain. Over the circumpolar region per petual snow covers the earth to the depth of many feet, and on the highland areas it accu mulates to so great a depth that the pressure of the superincumbent mass above causes a grad ual metamorphism in the character of the mass to take place, by which it is converted into glacial ice. Below 80° of latitude for each hemisphere (the general limit of perpetual snow) snow occurs in general in ever decreas ing quantities as the equator is approached, until at it is rarely seen at sea level. Over the northern portion of the temperate zones, and extending equatorward from 80° to about 45°, snow commonly covers the ground continuously during the winter months, and serves a most useful purpose in the econ omy of nature, in protecting vegetation and the earth's surface from the intense cold of winter. Owing to many causes, both meteorological and topographical, the amount of snowfall varies greatly at different localities, even as regards those situated upon the same parallels of lati tude. Upon the tops of high mountains, whether these be situated within the frigid, temperate or tropical zones, snow falls inter mittently throughout each year, and above a certain height upon them, called the snow-line, remains permanently, covering the summits. Owing to local causes, and to varying meteoro logical and other conditions, the snow-line upon mountains varies considerably in height, even as regards mountains situated upon the same parallels of latitude. In general it approxi mates 1,000 feet at 70°, 5,100 at 60°, 6,800 at 50°, 10,200 at 40°, 13,500 at 30°, 15,000 at and 10°, and 16,000 or 17,000 feet at the equator.
Two distinct varieties of snow occur,— the crystalline and the granular. The latter some times occurs as a distinct formation by itself, but more frequently as an accretion deposited by mist, or by clouds laden with minute rain drops, upon the crystalline varieties while these are passing through them on their earthward journey. Those particles wholly of a crystal line character are transparent like glass, vary in size from three-quarters to one-fiftieth of an inch in greater diameter, and fall to earth singly, or bunched together into flakes, accord ing as to whether the temperature and humidity are low or high. In general their size decreases in a certain ratio, with a decrease in the tem perature of the air, and vice versa. The ma jority exhibit a tendency to divide into six, due to the fact that snow crystals belong to the hexagonal system of crystallization.
The majority of snow crystals assume forms which, by virtue of outline structure or general characteristics, may be grouped into two principal classes,— the columnar and the tabular. Those comprising the former class are all such as appear in the form of hexa gonal columns, long, slender, needle-shaped crystals, or columns tapering at one end. The tabular snow-crystals develop upon an ex tremely thin tabular plane, whether they pos sess open or solid structure, or both combined. These two principal types often combine, tabu lar crystals forming upon one or both ends of the columnar crystals, on a plane perpendicular to their main axis, and so form compound crystals. • Crystallic growths sometimes take place from minute granular snow, the latter forming the nucleii, from which crystallic additions project or ramify in many planes. During ex treme cold, or when snowfalls emanate wholly from high cirro-stratus clouds, minute sub crystalline varieties often predominate, exhibit ing an imperfect or segmentary semblance to tabular or columnar forms.