SOAP

Snow Line, or Limit of Perpetual Snow. — Since the temperature of the atmosphere continually diminishes iu ascending from the surface into the higher altitudes, there must be in every latitude a certain limit of elevation at which the air attains the temperature of freezing water. This limit is called the snow line, or line of perpetual congelation; and the mountains which rise above it are covered with perpetual snow. Within the tropics the temperature varies little throughout the year, and hence the snow line is distinctly marked; but in countries remote from the equator the limit of congelation rises in summer and descends in winter through a band or zone of considerable breadth. The \me oi perpetual congeldXiow is of course the summer limit. The altitude of the snow line, however, is not dependent upon latitude alone; but also on the configuration and aspects of the mountain chains, the extent and temperature of the surrounding plains, the quantity of snow that falls annually, and the multitude of causes which influence the climate of a country. {See Climate.) On the Himalaya chain, for example, the limit of perpetual congelation on the northern side is at an elevation exceeding by upwards of 4400 feet that on the southern side, though the latter is directly exposed to the sun; a circumstance which Humboldt ascribes to the radiation from the great plain of Thibet, the general serenity of the climate, and the rarity of snow in an exceedingly cold and dry atmosphere. No dependence, therefore, can be placed on any general rule fores, timating the height of the snow line, and it affords an exceedingly fallacious indication of the altitude of mountains. The following table is given by Humboldt in his Fragmens Asiatiqucs, p. 549. The elevations stated are the lieights of the line of perpetual congelation above the level of the sea, and the measures are reduced to English feet. Mountain Chains. LaUtude. Elevation. Feet. Cordilleras of Quito Cordilferas of Bolivia 0° _• 1*0 S. 16 -17| 15,730 17,060 Cordilleras of Mexico 19 N. 15,030 Him=i»v= fNorthtrnside -\ Himalaya (southern side -) 31 f 16,940 I 12,470 Pyrenees 43 8,950 Caucasus 4.-? 10,870 Swi Alps 46 8,760 Car path i ans 49 8,500 Altai range - - 50 6,395 N--y - -: 70 3,518 2.302 (See, in addition to the works already cited, Humboldt, Memoire sur la Limite de Neige perpetuelle dans les Montagnes de r Himalaya, et les Regions Equator eales, in the A/males de Chimie, tome xiv.) [s. 1139]