The tropopause is the boundary between the troposphere (the lowest layer of the earth's atmosphere) and the stratosphere. It is the point at which the air ceases to become colder with increased altitude and also at which it becomes devoid of water vapor and dry. The tropopause's altitude is roughly 17 kilometers above the earth's surface in the equatorial regions and about nine kilometers above the surface in the polar regions, although it varies considerably according to the latitude and season and also daily.