Snow, the flocculent white masses of crystals in which the aqueous vapor of the atmosphere at low tempera-tares is precipitated from the clouds. The other forms in which atmospheric vapor appears are treated of under Dew, Frost, Hail, and Rain. The primary condition necessary to the formation of snow is the saturation of the air at a freezing temperature with vapor; the exact limits of temperature are not known, but probably vary with the density of the air and the vapor; the surplus vapor is precipitated from its invisible state in minute crystals, the primary form of which is that of a rhomboid having angles of 60° and 120°. (See Crys-tallography.) By far the larger part of snow falls during the night, and in many localities the maximum fall is between 1 and 7 A. M., which suggests that the cooling necessary to the production of snow is mainly due to radiation; a secondary maximum between 8 and 10 A. M. is explicable as due to the influence of the dynamic cooling of rising currents. The complexity of the forms of snow flakes increases with the quantity of moisture in the air, and probably with the variety of alternations of temperature to which they are exposed.
Their size increases with the temperature and humidity; thus they are much larger from 9 to 11 A. M. than before sunrise. Little however is satisfactorily known on these points. More than 1,000 forms of snow crystals have been observed and figured by Scoresby, Glaisher, Green, Stephen Lowe, and others. A very beautiful contribution to this subject was published anonymously in New York in 1863, under the title of "Cloud Crystals," in which over 150 new forms are added to those described by previous authors, and several interesting observations are given upon the conditions of their formation. The accompanying figures, representing specimens of the simple and the more complicated forms of crystals, are from Bu-chan's " Meteorology." Scoresby, who first studied these forms, classified them into : 1, thin plates (figs. 1 to 7); 2, spherical nuclei studded with needles (fig. 8); 3, three- or six-sided prisms or needles (fig. 9); 4, six-sided pyramids (fig. 10); 5, prisms terminated by planes (fig. 11). The conditions regulating the occurrence of each figure are probably quite definite, inasmuch as it is rare that more than three or four kinds of flakes occur at the same time.
The high cirrus clouds are probably generally formed of spicules, or possibly small flakes of snow, which when the clouds are not too thick give rise to the phenomena of halos (see Halo), and the geometrical explanation of these latter seems generally to require that the snow flakes present in these clouds should be principally of the simplest forms. The amount of snowfall in different parts of the earth is known with less accuracy than is that of rainfall, owing partly to the drifting of the snow, but especially to the fact that a too great diversity has existed in the methods adopted by the observers to ascertain either the quantity or the depth of the snow. It is generally assumed that 1/10 or 1/12 of the depth of snow measured immediately after falling will give the corresponding depth of melted snow. Quetelet, as the mean of many observations, says 1/9, but for very dry or very wet snow these fractions are very uncertain. The total depth of snowfall, is greatest, other conditions being the same, where the strong winds of winter are laden with moisture;' thus it averages annually 4 to 7 ft. in the interior of Maine, Vermont, New York, and Upper Canada, but only 2 ft. for the states in the same latitude further west.
One of the heaviest snowfalls recorded in America was that which continued from Feb. 19 to 24, 1717, when the snow remained 5 or G ft. deep over all the settled parts of New England. The geographical distribution of snow at sea level is such that in general in the eastern parts of North America and Asia it is rarely seen S. of lat. 30°, and in western Asia S. of lat. 36°. On the W. side of North America it is rarely seen at the sea level on the immediate coast, but is quite common in the interior. - Falls of snow may occur in any month in extreme polar latitudes; in New England and Canada snow falls mostly from November to March inclusive, but in the latitude of Washington, D. C, it falls mostly during January and February. The average number of days on which snow falls is, for St. Petersburg, 170; Paris, 12; Washington, D. C, 20; Gibraltar, 0; San Francisco, 0; Charleston, S. C, 1. But on ascending above the sea level we soon come to altitudes such that snow may fall and remain on the ground at any season; the altitude at which accumulations remain throughout the year is called the limit of perpetual snow.
The conditions governing this lower limit were first studied carefully by Humboldt in his climatology of Asia, and more recently has been investigated by Grad (1873); according to these, the limit in question has a general apparent connection with the isotherms of 32° F., but departs therefrom to an important degree when the prevailing winds are dry or moist. Thus the limit is lower in the southern than in the northern hemisphere; lower on the S. than on the N. side of the Himalaya mountains; lower within the tropics than under the latitudes 20° to 35°. From these latitudes it diminishes, according to Grad, to about 3,000 ft. in lat. 00° S. and 65° N.; but only in the high polar regions is the limit below 1,000 ft., it being higher in Greenland or Spitzbergen, where it is only the glaciers that descend to sea level. (See Glacier.) - Owing to the innumerable reflecting facets of the minute crystals and the quantity of air caught between the crystals, a layer of snow is a remarkably perfect non-conductor of heat; for this reason the covering of snow on the ground forms an almost perfect protection to the plants beneath against the freezing that would otherwise follow the radiation of their heat into the atmosphere.
In Ebermayer's "Influence of Forests" (1873) a case is quoted (by no means an extreme one) in which the temperature of the air was - 6.8° F., and that of the surface of the earth beneath the snow + 33.8° F., while below the surface the earth was still warmer. On the other hand, the individual crystals of snow have probably the same large radiating power as ice in larger solid blocks, which according to Leslie is 85, that of lampblack being 100. The consequence of this is, that during the night very hard frozen crusts are formed on the surface of the snow which has been somewhat thawed during the day; the same property, together with that of regelation, explains the peculiar structure of the surface snows of glaciers, and assists in the formation of areas of colder air over snow fields than over bare land. Equally important is the great absorptive power of snow for solar heat, since by reason of it the surface of a layer of snow is melted rapidly, and a large amount of moisture is thrown into the air, giving rise to extensive fog and haze, and having a decided influence on the development of storms. - Snow flakes in falling bring with them nearly all the fine dust floating in the air, leaving the atmosphere extremely pure; thus in northern Europe Nordens-kiold has found freshly fallen, snow impregnated with a black dust of carbon and iron such as could only have come from meteors; at other times the dust is such as could only have come from eruptions of volcanoes, especially those in Iceland. - Snow is occasionally tinged black, yellow, red, or green, as was known to Pliny. These colors are due to the presence of microscopic organisms, as was suspected by De Saussure (1700), which were described by Dr. Wollaston as minute spherical globules having a transparent covering and divided into seven or eight cells tilled with a red oily-like liquid insoluble in water.
Girod-Chantraus (1797 and 1802) described these as plants under the name volvox lacus-tris. Bauer (1820) demonstrated that they are a fungous growth, which he named uredo nivalis. Robert Brown concluded them to be alga) allied to the tremella cruenta. Agardh confirmed the views as to their vegetable nature, and gave them the title protococcus per-mesina. Bravais and Martins, as members of the northern commission, verified the identity of red (hcematococcus nivalis) and green (protococcus viridis) globules as being one and the same plants in different stages of growth, the green being probably the riper. The most recent authority on this difficult subject is Ros-tufinski (1875), who retains the generic name hoematococcus, and has farther confirmed the identity of these microscopic alga). Ehrenberg (1847) found, besides vegetable spores, animalcules properly so called, among which the most abundant in red snow is that to which he gave the name philodina roseola. - The glare of the sunlight reflected from snow-covered ground gives rise, unless the eyes are protected by glasses or goggles, to a very severe inflammation of the optic nerve. (See Amaurosis, and Blind).