School of Mathematics and Natural Sciences

# The Troposphere

## where we live in ...

In the picture of the „structural level“ of the atmosphere the troposphere is the first floor, our living space. This layer contains 90% of the total mass of the atmosphere.

The Earth's surface has impact on the near-surface layer of the troposphere. In addition to friction in this layer prominent physical processes take place which are of significance for the whole atmosphere. Therefore, this layer is called the atmospheric or planetary boundary layer (PBL). The whole energy exchange between the surface and the atmosphere occurs here.

The heat stored by the earth by the absorption of solar radiation passes into the air in contact with it. By turbulence and convection the heat is relatively quickly carried forward to the lower and later on to the upper atmospheric layers. The air gets mixed. The distribution of water vapor and the propagation of dust and other pollutants happens accordingly. The troposphere owes its denotation to the existence of permanent vertical and horizontal agitation and exchange processes (from Greek τροπή, tropé „turn, reversement“ and σφαίρα, sfaira „sphere“).

Due to the circulation and mixing in the troposphere this layer is characterised by a mean decrease in temperature of 6.5 K/km. The height of the troposphere fluctuates as a result of the different heating of the Earth's surface. The stronger the heating the thicker is the tropospheric layer. The position of the sun indirectly determines the mean temperature and the thickness of that layer. Thus, the troposphere achieves a mean height of 7-9 km at the poles (with a mean temperature at the top of -50°C) and 16-17 km at the equator (-80°C at the top). Temperature differences between summer and winter effectuate a 2 km lower top of the troposphere (defined as the tropopause) during winter.

The tropopause region is very sensitive to any kind of emissions. Here, a lot of trace gases evolve their highest climate effectivity. The tropopause is no impermeable and uniform layer. In fact the tropopause shows considerable discontinuances and sometimes complex structures, especially in the vicinity of the jet streams (polar and subtropical). The World Meteorological Organisation has endeavored to establish a more uniform definition of the tropopause. By definition, the tropopause is that height with an atmospheric pressure below 500 hPa from which the vertical temperature decrease falls to ≤2 K/km and this value is not exceeded in the overlying 2 km. Is a first tropopause defined and there is a second temperature decrease of more than 3 K/km within the next 1 km, a second tropopause is defined once the above criterion is fulfilled.

In conjunction with the fact that the troposphere contains almost the whole water vapor of the atmosphere (the most important greenhouse gas) all weather events which are linked to water vapor take place here. The formation of clouds and precipitation cleanse the troposphere of water-soluble gases and solids.

The water vapor exhibits a strong decline from the Earth's surface with increasing height. At the surface incoming radiative energy is absorbed after a short distance and is reemitted. With decreasing concentration of water vapor the mean free path increases. Thus the absorption lengths grow and the energy is radiated towards space. In that height region (i.e. approximately the tropopause region) the maximum of the radiative cooling is present. This causes a heat sink in the "water vapor sphere".

The troposphere can furthermore be seen as a chemical reactor which processes enormous amounts of trace gases. The concentrations, the spatial distributions and the temporal formation of trace gases are dependent on a sensitive equilibrium between emission, atmospheric transport and chemical self purification as well as dry and wet deposition.

An additional characteristic of the troposphere is an increase in the mean wind velocity with increasing height with its maximum near the tropopause. Here the mentioned jet streams are located which are willingly used by aircraft.