Atmospheric Layers
The vertical distribution of temperature can be used for classifying the different
atmospheric layers (the troposphere for heights below 8 km above the polar regions, and below 18 km above the Equator.The temperature is a decreasing function of altitude, down to 220 K above the polar regions and to 190 K above the Equator. The averaged temperature gradient is about ?6.5Kkm?1.
• then the stratosphere for heights up to 50 km.
The temperature is first constant and then an increasing function of altitude, up to about 270 K. This heating is directly related to the absorption of the ultraviolet solar radiation (UV) by ozone (O3) and by molecular oxygen (O2)
. This inversion layer is a specific property of the Earth’s atmosphere.
• then the mesosphere up to 85–90 km.
The temperature is a decreasing function of altitude down to 170 K (the coldest
atmospheric temperature), due to the rarefaction of ozone and of oxygen.
• and then the thermosphere and the ionosphere (up to about 150 km).
The temperature increases and is more and more dependent on solar activity.
The UV radiations dissociate N2 and O2 and gas-phase molecules are ionized
Air becomes a rarefied gas: the air density is about 1019 moleculem?3 at 100 km, to be compared with 1025 moleculem?3 at sea level • Beyond, the Earth’s attraction can be neglected. In the exosphere (at about
500 km), atomic hydrogen can escape from the atmosphere
There are two inversion layers in the atmosphere, characterized by a positive
gradient of temperature: in the stratosphere and in the ionosphere. Part of the solar
radiation is absorbed by a few gas-phase species in these layers, playing a filtering
role, which results in an increasing temperature. The vertical distribution of these
gas-phase species determines the vertical distribution of temperature.