Middle atmosphere
The atmosphere is divided into regions based on its temperature structure. Upward from the ground, these are
- troposphere
- stratosphere
- mesosphere and
- thermosphere
The 'middle atmosphere' refers to the region from the tropopause (10-16 km) to about 100 km. Above 100 km, the atmosphere is no longer well mixed, and ionisation becomes important.
Ionisation of atoms and molecules, by extreme ultra-violet radiation (an energetic component of sunlight that is outside the range of human vision) above 100 km heats the thermosphere to very high temperatures.
Ozone, which is prevalent in the stratosphere, absorbs UV-B radiation (a slightly less energetic component of sunlight, still outside the range of human vision) and heats the lower mesosphere and upper stratosphere.
Infra-red radiation (low energy electromagnetic radiation that is also outside the range of human vision) from the Earth heats the troposphere from below so that, near the ground, the temperature generally falls with altitude.
The polar upper mesosphere is significantly colder in summer than in winter. Pole-to-pole circulation at this altitude results in lifting and thus cooling of the region in summer, and subsidence and hence warming in winter.
The summer polar mesopause is the coldest region of the Earth's atmosphere, reaching temperatures as low as -140°C. It is sufficiently cold for noctilucent ('night shining') clouds to form in summer, at altitudes around 83 km.
The Antarctic stratosphere around 25 km altitude is cold enough for clouds to form in spring. Stratospheric clouds provide surfaces on which chlorine from CFCs catalytically destroys ozone in the presence of sunlight.
Increased concentrations of greenhouse gases such as carbon dioxide result in higher temperatures in the lowest reaches of the atmosphere, but cooling of greater magnitude in the upper atmosphere. The swap-over from heating to cooling depends principally on atmospheric density and occurs at about the height of the tropopause (10-20 km).
Some measurements of the middle atmosphere have reported cooling rates 5 to 10 times greater than expected from our present understanding.
The polar upper atmospheres may be regions where 'greenhouse warming' can be most definitively demonstrated. Whether this is so depends on the magnitude of the cooling compared to the natural variability. This is a subject of Australian Antarctic Division study.
