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?_

Because the _upper regions_ of the atmosphere are _intensely cold_.

444. _Why are the upper regions of the atmosphere intensely cold?_

Because the _atmosphere_ retains but _little of the heat of the sun's rays_ as they pass to the earth. Because at high altitudes the air is _greatly rarefied_. And because the _radiation of heat from the earth_ does not materially affect such _high regions_.

[Verse: "He causeth the vapours to ascend from the ends of the earth: he maketh lightnings for the rain: he bringeth the wind out of his treasuries."--PS. XXXV.]

445. _What is meant by the snow line?_

The _snow line_ is the estimated altitude in _all countries_ where _snow would be formed_. Even at the equator, at an altitude of 15,000 to 16,000 feet from the level of the sea, snow is found upon the mountain summits, where it perpetually lies. As we proceed north or south from the equator the _snow line lessens in altitude_. Had we in England a mountain 6,000 feet high, it would be perpetually _crowned with snow_.

446. _Why do we hear of red snow?_

Red snow is the name given to the snow in the arctic regions upon which a minute vegetable (probably the _Protoccus nivalis_) grows, imparting to the snow a red colour. Recent microscopic investigations have shown it to consist of a minute vegetable cell, which secretes a red colouring matter.

Snow is found to be of greater importance to man than is generally supposed. But, although in this country we are enabled to recognise the hand of Providence in the gift, there are latitudes wherein the blessing thus conferred is more deeply felt. In such countries as Canada, Sweden, and Russia, the falling of snow is looked for with glad anticipations, quite equalling those which herald the "harvest-home" of England, or the "vintage" of France. No sooner is the ground covered with snow, than cranky old vehicles that had been jolting over rough roads, and sticking fast in deep ruts of mud, are wheeled aside, and swift sledges take their place. Towns distant from each other find an easy mode of communication; the markets are enlivened, and trade thrives. Snow supplies a kind of railroad, covering the entire face of the country, and sledges glide over it, almost with the speed of the locomotive.

447. _What is sleet?_

_Sleet_ is snow which, in falling, has met with a _warmer current of air_ than that in which it congealed. It therefore partially melts and forms a kind of _wet snow_.

448. _What is hail?_

_Hail_ is also the _frozen moisture of the clouds_. It is probably formed by _rain drops_ in their descent to the earth, meeting with an _exceedingly cold current of air_ by which they become _suddenly frozen into hard masses_.

It is also supposed that the _electrical_ state of the air and of the clouds influences the formation of _hail_.

[Verse: "If the clouds be full of rain, they shall empty themselves upon the earth."--ECCLES. XI.]

449. _Why is it supposed that the electrical state of the air and the clouds affects the formation of hail?_

Because hail is more common in the _summer_ than at other seasons, and is frequently attended by storms of _thunder and lightning_.

450. _Why do hail-storms most frequently occur by day?_

Because the clouds, being charged with vapour to saturation, favour the formation of hail by _sudden_ electrical or atmospheric changes. In the gradual cooling of night, the clouds would expend themselves in rain.

Astonishing facts respecting hail-storms are upon record. In 1719 there fell at Kremo, hailstones weighing six pounds. In 1828 there was a fall of ice at Horsley, in Staffordshire, some of the pieces of which were three inches long, by one inch broad; and other solid pieces were about three inches in circumference. Hail storms are most frequent in June and July, and least frequent in April and October. Hail clouds float much lower in the sky than other clouds; their edges are marked by frequent heavy folds; and their lower edges are streaked with white, the other portions being massive and black. (Fig. 10.)