A gradual decrease in temperature takes place in the ascent from the sea
to the line of perpetual snow. This line, which is called the snow-line,
varies in different latitudes, and sometimes, owing to local causes,
differs on the same latitude; as a general rule, however, a gradual
decrease in elevation of the snow-line takes place as we recede from the
equator north and south. The height of this line within the tropics
varies from 16,000 to 17,000 feet above the level of the sea, and in the
northern hemisphere meets the level at about the eightieth parallel.