SNOW. It has been stated [RAIN, vol. vi. col. 923] that rain, snow, and hail are formed by the precipitation of vapour when two volumes of air of different temperatures, and saturated with moisture, become mixed together ; the nature of the precipitation depending on the resultant temperature and on that of the region of the atmosphere through which the aqueous particles descend towards the ground. Now when the precipitating water is frozen into crystals of ice, united together in such a manner as to reflect light to the eye in great abundance from all, thus producing a sensation of whiteness, the weremblagea of crystals constitute snow. In all probability it is formed by the immediate freezing of aqueous vapour, without the sensible intervention of the liquid state. Proximity to the earth's surface is not required for its production; Mr. Green, the wronaut, met with a severe snow-storm at the height of a mile and a quarter. The forma tion of snow through a considerable tract of the atmosphere appears at a distance as a bluish, sometimes indigo-blue, haze.
31. Menge observes (' Mnales de Chimie,' vol. v., p. 1), that the crystallisation of sal-ammoniac present(' phenomena similar to those which are observed in the formation of snow. If a saturated solution of sal ammoniac in a warm state be allowed to cool in a tranquil air, the surface of the liquid is that which first arrives at a state of super saturation, and there the first crystals are formed ; these pink imme diately, and in descending they unite with similar crystals formed in the liquid itself, so that they arrive at the bottom of the vessel in white flakes.
Tho flakes of snow usually consist of brilliant spicular icicles, which diverge from a centre in six directions, and resemble stars having so many rays, upon each of which email crystals are sometimes formed ; but if the atmosphere is agitated, the original flakes strike against each other, and uniting in groups by regelation [Ice], in consequence of small quantities of moisture adhering to them, they descend in irregular forms. In regions of the earth far to the north or south, the air, when allowed to enter through a small aperture into a heated apartment, has frequently caused the warm vapour to be converted into snow. (' Bibliotheque Universelle,' 1830.)
Becearia observed that his apparatus for ascertaining the electrical state of the atmosphere indicated the presence of electricity in snow as well as in rain ; and, according to the observations of Schtibler, it is more commonly positive than negative. The lightness of the flakes, by which they float about in the air when agitated, is the result of their surface being great when compared with their volume. The sp. gr. of snow has been stated to be very variable ; and according to Muss chenbreek, that of some, of the stelliform kind, was only of the specific gravity of water, but this must have been the specific gravity of the mass, a mixture in fact of air and ice ; and 3f. Quetelet has since 1 found that the greatest density is nearly of that of water, the tem perature being 34.5° (Fehr.). He ascertained also that the density of tine snow having no determinate form was about 1, the temperature being 32', and that the least density varied from to of that of water, at which time the snow had the form of small stars, and the temperature varied from 29.7° to Is.r. It is, however, difficult to understand these results ; it would appear that they also must relate to an aggregate of snow and air, as there is no reason why the specific gravity of crystals of snow should differ greatly from that of ice, which is always crystalline, and has the sp. gr. 0'918.
Snow has been observed to fall in it fine powder, not having any appearance of regular crystals, and sometimes in grains, as fine as those of what is called basket salt.
The flakes have, even in temperate regions, many varieties of form, and are often very elegant ; but the polar regions of the earth are those in which it has been supposed that nature has displayed her power in creating this species of beauty in the highest degree and to the greatest extent. In the' Phil. Trans ,' 1775, may be seen numerous delineations of the figures assumed by flakes of snow as they were observed by Dr. Nettis, of Middelburg in 1740; but the late Rev. Dr. Scoresby, in his ' Account of the Arctic Regions,' has given still greater varieties ; the latter gentleman, besides dividing them into classes, has also expressed their magnitudes, and the state of the barometer and thermometer when the snow fell.