=The snow-line.=—Because of the low temperatures glaciers should be most abundant or most extensive in high latitudes and in high altitudes. The largest are found in polar and subpolar regions, and they are elsewhere encountered only at considerable elevations. The largest glaciers are the vast sheets of ice which inwrap the continents of Greenland and Antarctica, but glaciers of large size are to be found upon other large land masses of the Arctic, as well as in Alaska, in the southern Andes, and in New Zealand. Much smaller glaciers are characteristic of certain highlands within temperate and tropical regions, but because of specially favorable conditions both of altitude and precipitation the Himalayas, although in relatively low latitudes, nourish glaciers of large proportions. In general, it may be said that the nourishing grounds of glaciers are largely restricted to those areas where snow covers the ground throughout the year. The lower margin of such areas is designated the _snow line_, and varies but little from the line on which the average summer temperature is at the freezing point of water—the so-called _summer isotherm of 32° Fahrenheit_. Within the tropics this line may rise as high as 18,000 feet above the sea, whereas in polar latitudes it descends to sea level.
=Importance of mountain barriers in initiating glaciers.=—The precipitation within any district depends, however, not alone upon the amount of moisture which is brought to it in the clouds, but upon the amount which is abstracted before the clouds have passed over it. The capacity of space to hold moisture increases with its temperature, and hence any lowering of this temperature will reduce the capacity. If lowered sufficiently, the point of complete saturation will be reached and further cooling must result in precipitation. Hence, anything which forces an air current to rise into more rarefied zones above, will lower the pressure upon it and so bring about a cooling effect in which no heat is abstracted. This so-called _adiabatic refrigeration_ of a gas may be illustrated by the cool current which issues in a jet from a warm expanded rubber tire after the cock has been opened; or even better, by the instant solidification at extreme low temperatures of such normal gases as carbonic acid when they are allowed to issue under heavy pressure from a small orifice.