The tropopause (from the Greek tropé “turn, turn” and pauein “end”) is the most important boundary surface of the earth's atmosphere and lies at a height of 6 to 18 km depending on the width . It is characterized by a clear change in the temperature profile and separates the troposphere , which is shaped by the weather , from the overlying, always stably stratified and very dry stratosphere .
Definitions and course
In the troposphere, the air temperature falls with altitude - apart from inversion positions , particularly unstable conditions and foehn situations - by around 0.5 to 0.75 ° C per 100 m (see also standard atmosphere and barometric altitude formula ). The stratification is stable as soon as the actual temperature gradient falls below the adiabatic one . Above the troposphere, the temperature initially remains almost constant at below −50 ° C. The temperature in the tropopause depends on the altitude, the lowest global temperatures of down to −80 ° C occur above the equator .
With the simplest definition of the tropopause, "reversal of the temperature gradient", smaller waves in real vertical temperature profiles would lead to ambiguities. The definition of the World Meteorological Organization is based on a gradient of −0.2 ° C per 100 m and tries to avoid ambiguities through additional conditions. With this gradient, the curvature of the temperature profile is typically greater, and the tropopause defined in this way is somewhat deeper, closer to the weather.
Alternative definitions use properties of air masses to determine their stratospheric or tropospheric origin:
- The trace gas CO z. B. arises near the ground and is degraded in the stratosphere on a time scale of months.
- Ozone, on the other hand, arises in the stratosphere and is broken down in the free troposphere (not to be confused with the local formation of ozone during summer smog ).
- A similar indicator of stratospheric origin is the amount of potential vorticity that is diminished in the troposphere by dissipation . Various numerical values, often 1.5 or 2 PVU , are used to define the tropopause ( this altitude diverges at the equator , there, for example, the potential temperature is used).
The various definitions of the tropopause yield an essentially matching meridional altitude course: flat in about 16 km altitude near the equator (about ± 20 degrees of latitude around which seasonal Shifting intertropical convergence zone around), then north and south until much, then again flat sloping up 6 to 9 km beyond 60 ° latitude, lower in winter than in summer.
Deviations from this are regional, such as an elevation in the West Pacific , or temporary, such as increases up to more than 18 km over Southeast Asia during monsoons . In the northern winter , cold air advances from the polar region often result in waste from 7 to 5 km to mid-latitudes, in extreme cases even deeper. The decline does not always run steadily: on the subtropical jet streams there are jumps downwards in connection with ambiguities (there tropospheric air then becomes more stratospheric), on the polar jet streams lowering of the tropopause occurs, occasionally with the entry of stratospheric air into the troposphere . See also Planetary Circulation .
In the interplay of global warming and ozone depletion , which lead to a rising temperature in the troposphere and a falling temperature in the stratosphere, a shift in the tropopause of several hundred meters in height between 1979 and 1999 could be determined.
Meaning of the border above the clouds
Since water is hardly present as water vapor at the low temperatures of the upper troposphere and its sedimentation as ice in the stable stratified lower stratosphere does not counteract convective mixing, it remains trapped in the troposphere. As a result, the atmosphere above the tropopause is very dry and there are practically no more clouds , which flight passengers often notice as fantastic views . The location of the tropopause can be recognized again and again by high, thin ice clouds ( feather clouds ) and often by the horizontal extent of the uppermost thunderclouds . Particularly strong updrafts cause some thunderstorms to shoot beyond the tropopause and carry small amounts of frozen water with their ice umbrella, the anvil , into the lower stratosphere.
The aridity of the stratosphere means that the thermal radiation emitted by water vapor in the upper troposphere escapes unhindered into space . The resulting cooling ensures convection below the tropopause on the one hand, and stability above on the other. This stable stratification of the stratosphere in turn prevents the transport of water and thus the loss of light hydrogen into space (above the protective ozone layer , water is split by the sun's UV radiation ).
The temperature proportions are higher on Venus , so a lot of water could escape there ( sulfuric acid with its higher boiling point has the same conditions on Venus as water on Earth: it rains down in the tropopause; water, on the other hand, passes through the tropopause of Venus without hindrance) . As a result, Venus lost a large part of its water.
The tropopause was reached in 1901/1902 in the course of a spectacular balloon ascent to 10,800 meters by Reinhard Süring and Arthur Berson . Despite a good supply of oxygen, the two balloonists fainted between 10 and 11 km altitude , but just before they pulled the life-saving line to sink. When the air pressure had risen from only about 25% to almost 50% at an altitude of around 6 km, they woke up at the same time, were able to stabilize the rapid drop 2 km above the ground and bring about a smooth landing.
In May 1902, the meteorologists Richard Assmann - the boss of the above balloonists - and Léon-Philippe Teisserenc de Bort published simultaneously about the existence of a stratosphere above the troposphere. The balloon had not quite reached the tropopause, but the temperature measurements carried out confirmed those of a recording balloon that had ascended at the same time and had pushed into the stratosphere. In addition, the measurements confirmed countless measurements carried out by the two meteorologists in previous years, in which they had never fully trusted their measured values, since the heating of the measuring devices by solar radiation could also have been the cause of the measured temperature increase. The researchers were able to prove that the air temperature does not drop any further upwards.
- Berlin, cradle of stratospheric research
- Atmosphere in the mountain weather lexicon from AlpenBergwetter.com
- WMO definition of the tropopause ( memento of the original dated August 22, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
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