Tropopause

The tropopause (from the Greek trope " turn, bend " and pauein " quit " ) is the main interface of the Earth's atmosphere and is dependent on the width in 6 to 18 km altitude. It is characterized by a marked change in the temperature gradient and separates the troposphere influenced by the weather of the above it is always stably stratified and very dry stratosphere.

Definitions and History of the tropopause

In the troposphere, the air temperature decreases with altitude - apart from minor inversions - by about 0.5 to 0.7 ° C per 100 m ( see also standard atmosphere and barometric formula ). The stratification is stable when the actual gradient falls below the adiabatic gradient. In the temperature remains almost constant at first below -50 ° C. Smaller waves in real vertical temperature profiles would with the simplest definition of the tropopause, " reversal of the temperature gradient ," therefore lead to ambiguities.

The definition of the World Meteorological Organization is based on a gradient of 0.2 ° C per 100 m and tried by adding conditions to avoid possible ambiguities. In this gradient, the curvature of the temperature profile is typically larger and thus defined tropopause is slightly lower, closer to the weather patterns.

Alternative definitions using properties of air masses to determine their stratospheric - tropospheric or origin. The trace gas CO, for example, arises close to the ground and is broken down in the stratosphere on a time scale of months, while ozone is formed in the stratosphere and is degraded 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, which is reduced in the troposphere by dissipation. Various numerical values ​​, often 1.5 or 2 PVU be used to define the tropopause ( at the equator diverges this height, for example, there is then used the potential temperature ).

The various definitions of the tropopause result in a substantially coincident meridional height profile: flat in about 16 km altitude near the equator (about ± 20 degrees of latitude to the seasonally Shifting intertropical convergence zone around), then until well after the north and south, then flat sloping down to 6-9 kilometers beyond 60 ° latitudes in winter is lower than in the summer. Variations are regional, such as an elevation in the Western Pacific, or temporary, such as increases of up to 18 km over Southeast Asia monsoon periods. The waste also does not always steadily: On the subtropical jet streams come into contact with ambiguities jumps down in front ( there is then tropospheric air to stratospärischer ), occur at the polar jet streams lowering of the tropopause on, sometimes with retraction of stratospheric air into the troposphere. See also Planetary circulation.

The temperature at the tropopause depends on their height. The global lowest temperatures up to -80 ° C occur over the equator.

The interplay between global warming and ozone depletion, leading to an increasing temperature in the troposphere and a decreasing temperature in the stratosphere, a shift of the tropopause by several hundred meters 1979-1999 in the amount of 2003 could be found.

Importance of the border above the clouds

Since water at the low temperatures of the upper troposphere barely present as water vapor and its sedimentation counteracts as ice in the stably stratified lower stratosphere no convective mixing, it remains virtually trapped in the troposphere. Therefore, the atmosphere is very dry above the tropopause and there is virtually no clouds, what airline passengers often strikes as fantastic view. In the global external view visible, the position of the tropopause by widening some high clouds. Particularly violent updrafts allow some tropical storm over the surrounding tropopause shoot and carry small amounts of water into the lower stratosphere.

The dryness of the stratosphere results in that the radiation emitted by the water vapor in the upper troposphere heat radiation escapes freely into space. The consequent cooling ensures on the one hand (above the protective ozone layer is water split from the UV radiation of the sun for the convection below the tropopause, on the other hand above for stability, which in turn inhibits the transport of water and thus the loss of the light hydrogen into space ).

On Venus, the temperature proportions are higher, so much water could escape (sulfuric acid has the higher boiling point where the conditions like water on the earth, it rains from the tropopause, water passes freely there ). As a result, it has lost much of its water.

Discovery

The boundary layer to the tropopause was discovered in the years 1901/1902 as part of a spectacular balloon ascent to 10,800 feet by Reinhard Süring and Arthur Berson. The two balloonists fell despite good supply of oxygen 10 to 11 km altitude in a deep swoon, but attracted just before the lifesaving rope to sink. When the air pressure of only about 25 % in around 6 km altitude again was almost 50 %, they awoke at the same time, the rapid decline could stabilize 2 km above the ground and bring about a smooth landing.

In May 1902 published the meteorologist Richard Assmann - the head of the oe Balloonist - and Léon- Philippe Teisserenc de Bort at the same time about the existence of an overlying stratosphere. The balloon had the tropopause not quite reached, but the temperature measurements made confirmed those of a simultaneously ascended registration balloon was advanced into the stratosphere. The researchers were able to demonstrate the upward not sinking air temperature.