Stratosphere & Troposphere

The stratosphere (/ˈstrætəˌsfɪər, -toʊ-) is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. About 20% of the atmosphere's mass is contained in the stratosphere. The stratosphere is stratified in temperature, with warmer layers higher and cooler layers closer to the Earth. The increase of temperature with altitude, is a result of the absorption of the Sun's ultraviolet radiation by ozone. This is in contrast to the troposphere, near the Earth's surface, where temperatures decreases with altitude. The border between the troposphere and stratosphere, the tropopause, marks where this temperature inversion begins. Near the equator, the stratosphere starts at 18 km (59,000 ft; 11 mi); at mid latitudes, it starts at 10–13 km (33,000–43,000 ft; 6.2–8.1 mi) and ends at 50 km (160,000 ft; 31 mi); at the poles, it starts at about 8 km (26,000 ft; 5.0 mi). Temperatures vary within the stratosphere with the seasons, in particular with the polar night (winter). The greatest variation of temperature, takes place over the poles in the lower stratosphere; those variations are largely steady at lower latitudes and higher altitudes.

The troposphere is the lowest portion of Earth's atmosphere, and is also where all weather takes place. It contains approximately 75% of the atmosphere's mass and 99% of it is water vapor and aerosols. The average depths of the troposphere are 20 km (12 mi) in the tropics, 17 km (11 mi) in the mid latitudes, and 7 km (4.3 mi) in the polar regions in winter. The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km (1.2 mi) deep depending on the landform and time of day. Atop the troposphere is the tropopause, which is the border between the troposphere and stratosphere. The tropopause is an inversion layer, where the air temperature ceases to decrease with height and remains constant through its thickness.

The word troposphere derives from the Greek: tropos for "turn, turn toward, trope" and "-sphere" (as in, the Earth), reflecting the fact that rotational turbulent mixing plays an important role in the troposphere's structure and behaviour. Most of the phenomena we associate with day-to-day weather occur in the troposphere.