|The Earth atmosphere is a layer of gases surrounding our planet : This layer is retained by Earth's gravity. It is composed of "air", which contains 78% nitrogen, 21% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%.|
The atmosphere protects life on Earth by:
- absorbing ultraviolet solar radiation,
- warming the Earth surface through heat retention (greenhouse effect), and
- reducing temperature extremes between day and night.
In the atmosphere, we observe the processes which we call "Weather".
Atmosphere layers. Author: Niko Lang.
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and 99% of its water vapor and aerosols. It is the layer, where the "weather" occurs. The border between the troposphere and the next layer (stratosphere), is called the tropopause, is a temperature inversion.
The average heigth of the troposphere is approximately 17 km in the middle latitudes. It is higher in the tropical regions (up to 20 km), and shallower near the poles (up to 8 Km in summer, and 6-7 Km in winter). The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km deep, depending on the landform and time of day.
The troposphere is warmed up only to a small extent directly by sunlight. The largest part of the heat is taken up by from the ground, which warms up in the sun. The air temperature decreases from the ground on average by approximately 6.5 °C per kilometer height. This is the "vertical atmospheric temperature gradient".
Specifically, the temperature decreases on average by 10°C per Km altitude under dry-adiabatic (= cloudless) conditions (Dry adiabatic lapse rate = DALR), and about 6°C per Km altitude under the moist-adiabatic (cloudy or foggy) conditions (Saturated adiabatic lapse rate = SALR).
At the tropopause, the temperature is at approximately - 75 °C (at the equator) to - 45 °C (at the Poles). Since warm gases rise and cold air drops, it mixes within the troposphere, and this creates the "weather". At the ozone layer in the stratosphere, the temperature starts rising again with increasing height (inversion), and this represents a mixing boundary.
Therefore all processes, which affect the weather, occur in the troposphere: Heating leads to temperature differences and creates wind flows, clouds are formed by the existing water vapour, rains develop and clean the troposphere from solved gases and solids.
Temperature layers in the atmosphere
The temperature in the atmosphere decreases with the altitude.
Actually, warm air should rise. This does not happen, however - why? Since the density of air decreases with the altitude, the air in the upper layers is lighter than the underlying, warmer air layers - which are, however, denser because they are under higher air pressure. Therefore, the warm air does not rise - or does it nevertheless?
Sometimes, warm air rises if air is particularly heated and thus it becomes lighter: then, it displaces the colder air above, and ascends until it has expanded and cooled down so much that its weight is no longer lighter than that of the surrounding air.
An example of the temperature stratification of the atmosphere is shown in the picture to the right.