That portion of the atmosphere from the earth's surface to the tropopause; that is, the lowest 10–20 km (6–12 mi) of the atmosphere; the portion of the atmosphere where most weather occurs. The troposphere is characterized by decreasing temperature with height, appreciable vertical wind motion, appreciable water vapor, and weather. Dynamically, the troposphere can be divided into the following layers: surface boundary layer, Ekman layer, and free atmosphere. See atmospheric shell.