CHAPTER XXI.

440. What is snow?

Snow is congealed vapour, which would have formed rain; but, through the coldness of the air, has been frozen in its descent into crystalline forms. ([Fig. 1].)

441. Why is snow white?

Because it reflects all the component rays of light.

442. Why is snow said to be warm, while white garments are worn for coolness?

Snow is warm by virtue of its light and woolly texture. But it is also warm on account of its whiteness; for, had it been black, it would have absorbed the heat of the sun, which would have thawed the snow. Instead of which, it reflects heat; and the reflected heat falls upon bodies above the snow, while the warmth of the earth is preserved beneath it. White clothing is cool, because it reflects from the body of the wearer the heat of the sun. White snow is warm, because it reflects the sun's heat upon bodies.

There are few persons but have felt the effect of the sun's rays reflected by the white snow on a clear wintry day. And, as regards the warmth of snow towards the earth, by preventing the radiation of heat, it has been found that a thermometer buried four inches deep in snow has shown a temperature of nine degrees higher than at the surface.

443. Why are lofty mountains always covered with snow?