The Stratosphere
where everything is "stratified" ? ...
In the beginning of the 20th century above the troposphere a layer with nearly constant or slightly increasing temperature was detected with unmanned registering balloons. Such a temperature profile leads to a stable layer, i.e. a region poor in convective and exchange processes. Thus, this layer was denoted as the stratosphere (from Latin stratum „cover“ and Greek σφαίρα, sfaira „sphere“). Meanwhile, it is known that the imagination of a calm layer above the agitated troposphere is not really correct since also in the stratosphere changes in temperature and wind velocity of comparable amount as in the troposphere are present on short term and long term time scales.
Daily temperature changes in the stratosphere are almost of opposite sign in the troposphere with nearly the same value. I.e. a stratospheric warming implies a tropospheric cooling. This behavior is known as the counterpart principle or as the stratospheric compensation.
The constance or slight increase in temperature is due to the radiative cooling by water vapor in the upper troposphere, in the tropopause region and in the warming by absorption of solar irradiation in the stratosphere. The latter is effected by the trace (and greenhouse) gas ozone (a three-atomic oxygen molecule). Ozone is distributed in the atmosphere from the ground to approximately 50 km height. The layer with the maximum amount of ozone, the ozone layer, is located at 26 km height in middle latitudes. The stratospheric ozone layer provides a shield for the biologically damaging UV radiation of the sun.
Enhanced ozone concentrations have detrimental effects on fauna, flora and human organisms. Although the amount of ozone of several ppmv (parts per million by volume) is primarily present far above the tropopause it has a strong impact on the lower atmospheric layers through heating. The stratospheric temperature profile is thus mainly determined by the radiative equilibrium between solar irradiation and cooling by radiation in the infrared. The stronger temperature increase with height in the stratosphere occurs in the region of the ozone layer. In this region the temperature increases from -60°C to lower than 0°C in the mean.
The most prominent greenhouse gas, water vapor, is only apparent in the stratosphere with low volume concentrations. In tropical regions a lot of trace gases, inclusive water, are transported into the stratosphere by huge convective systems. From there, they are distributed globally, as by a "circulation pump". By entering the stratosphere the air must cross the tropopause. Ice clouds form in the severe cold. The tropopause is much colder in the tropics compared to higher latitudes. The water is imprisoned in a cold trap on its way into the stratosphere. It is trapped in form of ice crystals. The air is freeze-dried. An almost dry air mass remains. Here it is determined how much water can enter the stratosphere.
An important implication is, that globally the water vapor content in the stratosphere is only about a few thousands of the values in the troposphere. Due to the very dry stratospheric air, clouds only form under very cold conditions there. Nevertheless, recent measurements show that the stratosphere must not always be so dry as previously assumed.
