The atmosphere is divided into regions.
Troposphere
This is where we live, grow crops, have our factories, houses and most scientific laboratories.
In the lowest 15 km of the atmosphere, temperature decreases with altitude. This happens at a nearly constant rate of approximately −6°C per km. This is where most of what we call ‘weather’ occurs. The troposphere is characterised by convective motion. Water vapour, aerosols and the ‘greenhouse gases’ dominate the climate processes.
Tropopause
The Tropopause is the upper boundary of the Troposphere. We are higher than Mt. Everest (8.8 km), and above most of the world’s weather (rain and clouds). Intercontinental passenger aircraft fly at this height.
Tropospheric temperatures maximise near the equator and decrease towards each pole. In this region, the temperature goes from decreasing with height, to increasing with height. This transition is more pronounced at the poles. The tropopause is highest in the tropics (~16 km) and lowest in the polar regions (~8 km).
The stratosphere and stratopause
You are now above 90% of the Earth’s atmosphere, measured by mass. The temperature is very low and the air is very thin — a human could not survive.
From the tropopause to about 50 km up, temperature increases with altitude. This region is the stratosphere. Only weak vertical motions of air occur. Most energy flow comes from radiative processes. The increase in temperature with altitude is due to heating by ozone absorption of solar ultraviolet (UV) radiation.
Stratospheric temperatures are highest over the summer pole and decrease steadily to a minimum over the winter pole.
At the top of the stratosphere, the temperature starts to decrease with altitude again. This region is the stratopause.
Like all atmospheric regions, the stratosphere contains aerosols. Aerosols play a major role in atmospheric chemistry. They contribute to the formation of clouds (through the nucleation of water droplets). They also contribute to the cooling of the atmosphere through re-radiation.
Aerosols in the stratosphere play a major role in the cooling of the lower atmosphere and assist in the destruction of ozone. Some stratospheric aerosols are natural (coming from sources such as meteors and volcanic events). The rest are of human origin, coming from sources such as CFC emissions and high altitude aircraft exhausts.
The Antarctic stratosphere is cold enough for clouds to form in spring. Stratospheric clouds provide surfaces on which chlorine from chlorofluorocarbons (CFCs) destroys ozone in the presence of sunlight.
The mesosphere
From the stratopause up to about 85 km (95 km during the winter), is the mesosphere. Temperature decreases with altitude as ozone heating diminishes. In this region, convective and wave motions and radiative processes are important in transporting energy.
Temperatures are coldest over the summer pole and increase steadily to a maximum over the winter pole. The region at the top of the mesosphere is the mesopause.
The mesopause
The mesopause is the coldest region of the Earth’s atmosphere (ranging between −150°C in winter and −90°C in summer).
It is cold enough for noctilucent (‘night shining’) clouds to form in summer, at altitudes around 83 km. The altitude of the mesopause varies with season and latitude. In the polar regions, the altitude of the mesopause ranges between 90 km in summer and 110 km in winter. Atmospheric physicists suspect the mesopause is very sensitive to climate change. Climate change variations are warming the troposphere and this is thought to be causing further cooling in the mesosphere.
The thermosphere
The region above the mesopause is called the thermosphere.
Thermospheric temperatures are strongly influenced by solar activity. At this height and above, the atmosphere is extremely thin (very low density). The bottom of the green auroral light comes from near this height.
