2. The atmosphere

2.1 Structure

The atmosphere can be considered as a series of layers concentric with the earth, delimiting several zones.

The two main zones are :

• the homosphere, located from the ground to an altitude of about 80 to 90 km ;

• the heterosphere, beyond.

Within the homosphere, three main atmospheric layers are defined, differentiated by their temperature gradient with altitude (figure 3 ):

• the troposphere ;

• the stratosphere;

• ...