Structure of the atmosphere
The atmosphere, the earth’s gas envelope, consists mainly of nitrogen (~78%) and oxygen (~21%), water vapour and other gases. The diagram shown below presents the average temperature curve of the atmosphere and indicates clearly that the atmosphere consists of different layers with different characteristics.
The troposphere contains by far the greatest proportion of the atmospheric mass. Because the air masses are heated by the heat emitted by the earth’s surface, the temperature declines on average with increasing altitude (and hence increasing distance from the earth’s surface). Because cold air is more dense and hence heavier than warm air with the same volume, the cold packets of air sink while the air masses heated by the earth’s surface rise. This turbulent exchange of air masses between higher and lower layers of air is known as convection. Convection is one of the important properties of the troposphere which is responsible for the fact that weather activity mainly unfolds in this layer. The troposphere is bounded from above at a height of around 10-15 km by the tropopause. Between this boundary and the stratosphere, the temperature increases again (at -50°C here); this gives rise to a stable stratification in which vertical movements of air masses are quickly halted.
The temperature in the stratosphere increases with altitude. The reason for this is the formation of ozone through highly-energetic UV radiation (photodissociation) which heats the atmosphere in this layer. Above the stratosphere (from around 50 km in height), the temperature declines significantly in the mesosphere due to the strongly declining concentration of the air masses and then it rises again significantly in the ionosphere and exosphere.
As a result of the unequal distribution of ocean areas and land masses, the rotation of the earth and angle of the earth’s rotation axis, characteristic patterns of atmospheric circulation arise that control and drive the horizontal exchange of air masses.