Although air is well mixed throughout the atmosphere, the atmosphere itself is not physically uniform but has significant variations in temperature and pressure with altitude, which define a number of atmospheric layers. These include the troposphere (0 to 16 km), stratosphere (16 to 50 km), mesosphere (50 to 80km) and thermosphere (80 to 640km). The boundaries between these four layers are defined by abrupt changes in temperature, and include respectively the tropopause, stratopause and mesopause. In the troposphere and mesosphere, temperature generally falls with increasing altitude, whilst in the stratosphere and thermosphere, temperature rises with increasing altitude.
In addition to temperature, other criteria can be used to define different layers in the atmosphere. The ionosphere, for example, which occupies the same region of the atmosphere as the thermosphere, is defined by the presence of ions, a physico-chemical criterion. The region beyond the ionosphere is known as the exosphere. The ionosphere and the exosphere together make up the upper atmosphere (or thermosphere). The magnetosphere is the region above the Earth's surface in which charged particles are affected by the Earth's magnetic field.
Another well-known layer of the atmosphere is the ozone layer, occupying much of the stratosphere. This layer is defined by its chemical composition - where ozone is especially abundant.