The troposphere [ tropoˈsfɛːrə ] (from ancient Greek τροπή tropé "twist, change" and σφαίρα sphaira "sphere") is the lowest layer of the earth's atmosphere and part of the homosphere . The troposphere extends from the ground to the beginning of the stratosphere . The boundary in between is called the tropopause .
Its thickness is about 8 kilometers at the poles (where it is up to 2 kilometers lower in winter than in summer) and 18 kilometers at the equator . The troposphere contains around 90 percent of all air and almost all of the water vapor in the earth's atmosphere. Since the majority of the weather takes place in it, one also speaks of the weather layer (or advection layer ) of the atmosphere.
The troposphere is only heated to a small extent by direct sunlight. Most of the solar energy is converted on the ground and released into the atmosphere, which is why it is warmest near the ground. Since rising air expands and cools down, the air temperature decreases with increasing altitude by an average of 6.5 K per kilometer of altitude (definition of the standard atmosphere ). This is called the vertical atmospheric temperature gradient .
In detail, the temperature decrease in dry adiabatic (= cloudless) sections is on average 1 degree per 100 meters ( DALR ), in humid adiabatic (cloud and fog-rich) areas it is about 0.6 degrees ( SALR ) per 100 meters . At the tropopause the temperature is around -75 ° C (at the equator) to -45 ° C (at the poles). From this altitude on, the temperature initially remains the same ( isothermal energy ) and then increases again in the stratosphere (inversion); at an altitude of 50 km it is again 0 degrees Celsius.
As warm gases rise and cold gases fall within the troposphere, the air is mixed and the formation of weather becomes possible. The existing water vapor forms clouds , rains and the troposphere is cleaned of dissolved gases and solids. Because of the inversion, the tropopause represents a mixing limit. Therefore, there is only a slight exchange of air with the stratosphere. All processes that influence the weather take place in the troposphere.
Outline of the troposphere
The troposphere consists of the following layers:
- Bottom centimeter: laminar lower layer . Is of particular interest for microclimatic issues, as there is practically no air movement in this layer due to the intense friction.
- Lowest approx. 2 meters: boundary layer close to the ground , also Geiger layer . A strong vertical increase in wind speed and, especially at high temperatures, also a strong temperature decrease with altitude. The boundary layer close to the ground is the habitat of humans, animals and largely also the vegetation.
- Lowest approx. 50 meters: air layer close to the ground , also Prandtl layer . The influence of the earth's surface is still important and there is a pronounced increase in wind speed with altitude (due to the decreasing ground friction). Much of the earth's vegetation protrudes into this layer.
- Lowest approx. 0.5–2 kilometers: planetary boundary layer , also called friction layer or peplosphere . In this outer layer, the influence of friction manifests itself primarily in a change in wind direction with altitude, as does a further increase in vertical wind speed.
- Above (up to the tropopause): free layer or free atmosphere . It is essentially free of relief friction. Most of the air traffic is handled in this shift.
According to another definition, the planetary boundary layer extends (almost) to the ground. It is then divided into an inner layer close to the ground ( Prandtl layer , below 50 m) and an outer layer ( Ekman layer , above 50 m). The Ekman layer is characterized by wind rotation and the increase in wind with height (see Ekman spiral ).
- CD Schönwiese: Climatology . Ulmer Verlag, Stuttgart 2003, ISBN 3-8252-1793-0
- The Troposphere , Bergische Universität Wuppertal, Atmospheric Physics