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.
"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.