The Earth's atmosphere is divided into several altitude regions.
These regions start and finish at varying heights depending on season and distance from the poles. The altitudes stated below are averages:
• Troposphere: surface to 8,000 metres (5.0 mi) at the poles, 18,000 metres (11 mi) at the Equator, ending at the Tropopause
• Stratosphere: Troposphere to 50 kilometres (31 mi)
• Mesosphere: Stratosphere to 85 kilometres (53 mi)
• Thermosphere: Mesosphere to 675 kilometres (419 mi)
• Exosphere: Thermosphere to 1,000 kilometres (6,200 mi).
At high altitude, atmospheric pressure is lower than that at sea level. This is due to two competing physical effects: gravity, which causes the air to be as close as possible to the ground; and the heat content of the air, which causes the molecules to bounce off each other and expand. Between 30 and 50 km of altitude the temperature increases again due to ozone.
The temperature profile of the atmosphere is a result of an interaction between radiation and convection. Sunlight in the visible spectrum hits the ground and heats it. The ground then heats the air at the surface. If radiation were the only way to transfer heat from the ground to space, the greenhouse effect of gases in the atmosphere would keep the ground at roughly 333 K (60 °C; 140 °F), and the temperature would decay exponentially with height.
However, when air is hot, it tends to expand, which lowers its density. Thus, hot air tends to rise and transfer heat upward. This is the process of convection. Convection comes to equilibrium when a parcel of air at a given altitude has the same density as its surroundings. Air is a poor conductor of heat, so a parcel of air will rise and fall without exchanging heat. This is known as an adiabatic process, which has a characteristic pressure-t. Note that only the troposphere (up to approximately 11 kilometres (36,000 ft) of altitude) in the Earth's atmosphere undergoes notable convection; in the stratosphere, there is little vertical convection.