Snow

(sno.) [A. S. snaw.) (Meteor.) A cong elation of vapor produced in the middle region of the air, when the temperature of the atmosphere sinks below the freezing-point of water. The manner in which S. is produced is not well understood. It has not been ascertained whether the clouds which give rise to it are composed of vesicular vapors, or frozen particles; nor whether the flakes are completely or only partially formed before it begins to descend; nor is it known what temperatures or circumstances give rise to its diff or ent appearance. Upon examining S. flakes they are found,with rare exceptions, to be ma es of beautifully formed crystals. Their variety is endle , but the principal forms are stars of six points, from one-third to one thirty-fifth of an inch in diameter (Fig. 598). S. plays an important part in the economy of nature. In the first place, the mere transformation of the water particles into ice is a proce during which a large amount of heat is given out; so that we may regard the formation of S. as a proce tending to render the air currents warmer than they would otherwise be. Then when the S. has fallen it serves to protect the ground, for, owing to its loose texture, it is a bad conductor of heat; so that, while checking the radiation of heat from the earth into space, it does not draw off the earth's heat by conduction. The ground is thus often 20° or 30° warmer than the surface of the Sabove it, and sometimes the difference of temperature has been more than 40°, Red S. and Green S. have been met with, more commonly in Arctic regions, but also in other parts of the world. These colors are caused by the presence of minute organisms -a species of alga called Pro to coccus nival is. The S.-line is the line on mountain slopes below which all the snow which falls in the year melts during the summer. Above the snowline, therefore, lies the region of perpetual snow. The altitude of the snow-line depends on a variety of conditions. The latitude of a mountain-range is, of course, important in determining the position of the snowline, but many other circumstances have to be considered, as the shape and slope of the mountain, the aspect of either side of the range, the character of the surrounding country, the prevalent winds, and so on. Fig. 598.-SNOW-CRYSTALS. [s. 834] IMG:p0879_0.jpg:Gravür