Temperature decreases with the height above the level of the sea, as well as with the latitude. The air in the higher regions of the atmosphere is much cooler than that below, because the warm air expands as it rises, by which its capacity for heat is increased, a great proportion becomes latent or absorbed, and less of it sensible. A portion of air at the surface of the earth whose temperature is 70° of Fahrenheit, if carried to the height of two miles and a half, would expand so much that its temperature would be reduced 50°; and in the ethereal regions the temperature is 239° below the zero point of Fahrenheit.
The height at which snow lies perpetually decreases from the equator to the poles, and is higher in summer than in winter; but it varies from many circumstances. Snow rarely falls when the cold is intense and the atmosphere dry. Extensive forests produce moisture by their evaporation; and high table-lands, on the contrary, dry and warm the air, because the air at great elevations is too rare to absorb much of the sun’s heat. In the Cordilleras of the Andes, plains of only twenty-five square leagues from their extent raise the temperature as much as 3° or 4° above what is found at the same altitude on the rapid declivity of a mountain, consequently the line of perpetual snow varies according as one or other of these causes prevails. Aspect in general has also a great influence; yet the line of perpetual snow is much higher on the northern than on the southern side of the Himalaya, partly because the air is nearly deprived of its moisture by precipitation before it arrives at the northern side of the mountains. On the whole, it appears that the mean height between the tropics at which the snow lies perpetually is about 15,207 feet above the level of the sea; whereas snow does not cover the ground continually at the level of the ocean till near the north pole. In the southern hemisphere, however, the cold is greater than in the northern. In Sandwich Land, between the 54th and 58th degrees of latitude, perpetual snow and ice extend to the sea-level; and in the island of S. Georgia, in the 53rd degree of south latitude, which corresponds with the latitude of the central counties of England, perpetual snow descends even to the level of the ocean. It has been shown that this excess of cold in the southern hemisphere cannot be attributed to the winter being longer than ours by 7-3/4 days. It is probably owing to the open sea surrounding the south pole, which permits the icebergs to descend to a lower latitude by 10° than they do in the northern hemisphere, on account of the numerous obstructions opposed to them by the islands and continents about the north pole. Icebergs from the Arctic seas seldom float farther to the south than the Azores; whereas those that come from the south pole descend to as low a latitude as that of the Cape of Good Hope.