What is the middle atmosphere?

The middle atmosphere is that part of Earth's atmosphere that lies above our weather and below the ionosphere (where the air is ionised by incoming solar radiation). It encompasses altitudes that are often referred to using more specific terms, as shown in the diagram below. 

Click on the name of the atmospheric layers for a description

At its base is the stratosphere, where the temperature increases with height. The water vapour that plays such an important role in our weather (in the troposphere) can't make it through the extreme cold at the base of the stratosphere, which means the region is quite dry. The increase in temperature with height, due to the absorption of energy by the ozone layer, makes the region very stable.

The mesosphere, which lies above the stratosphere, is a region where the temperature decreases with height. The atmosphere at this height is both tenuous (there is only 0.01% of the air here compared to the ground) and turbulent. The cold air overlying the warm air makes the atmosphere quite unstable in this region.

The lower part of the thermosphere also lies in the middle atmosphere. Highly energetic radiation from the sun heats the thermosphere to very high temperatures. It can also separate some of the electrons from the atoms and molecules of the air, making the region ionised. That part of the thermosphere that falls in the middle atmosphere is only weakly ionised and the ionisation does not greatly affect the processes that occur there.

Notice that between the 'spheres' described above there are 'pauses'. These are regions at the top of each 'sphere' where the characteristic that defines it gives way to that defining the region above it. For example, the stratopause is where the temperature goes from increasing with height (as in the stratosphere) to decreasing with height.

Temperature and wind in the middle atmosphere

In the early days of its exploration, the polar middle atmosphere showed that it was full of surprises. For example, the temperature structure of the mesosphere was found to be the opposite of what was predicted: extremely cold in the summer when bathed in sunlight and warmer than expected in the darkness of winter.

The reason behind this unexpected behaviour is the role of wind (dynamics) in the temperature structure in relation to warming of the air by the sun (radiation).

If the atmosphere is still, each of its constituent parcels of air is warmed by the transfer of radiation from the sun (which is obviously hotter than the air parcel) but cooled because it transfers infra-red radiation to space (which is cooler than the air parcel). If there is no sunlight, then the only warming source of radiation is the earth below, and in Antarctica, this is not very warm.

If the atmosphere is in motion, however, another mechanism for heating and cooling comes into play. When an atmospheric air parcel is forced upward into the less dense regions that lie above it (up-welling), it expands and makes the temperature of these upper levels decrease. This effect is similar to the cooling effect air experiences when it is let out of the higher pressure environment inside a car tyre. The opposite is true in the atmosphere if air is forced downward (down-welling). It is compressed and warms accordingly.

The dominant component of motion in the atmosphere is horizontal. However, when it was found that vertical motions exist in the middle atmosphere, the mystery of the cold mesosphere was unravelled. Up-welling in the summer polar mesosphere makes this region the coldest part of the terrestrial environment. And down-welling in the winter has a warming effect.

The roles of both radiation and dynamics in the polar middle atmosphere make attempts to measure both wind and temperature in this region vitally important in enhancing our understanding of the atmosphere. 

The atmospheric temperature and the wind speed are usually the two parts of the evening weather report that interest us. This is also the case in studies of the middle atmosphere climate. However, the remoteness of most of the middle atmosphere makes it difficult to measure these parameters and exotic tools like radars, spectrometers and LIDARs ('light detection and ranging' instruments) are needed. Satellites, balloons and rockets are also useful and can yield other information such as the concentration of trace chemical species.