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At the top of the stratosphere, the rise in temperature ceases. In the mesosphere (50–80 km) there is a continuous decrease of temperature, reaching a minimum of almost –100°C. The temperature then rises rapidly as the atmosphere becomes ever thinner in the thermosphere.

The upper part of the mesosphere and much of the thermosphere (80–500 km) is also known as the ionosphere. In this region, high‐energy UV and X‐ray solar radiation splits atoms and molecules – a process known as ionization. Radio stations take advantage of the fact that these free electrons and ions reflect radio waves.

The outermost region, above an altitude of about 500 km, is known as the exosphere. Its main constituents are the lightest gases, particularly hydrogen and helium, with some atomic oxygen. The exosphere is characterized by the escape of these gases into space (Figure 3.9). Air pressure is so low that it is effectively a vacuum. However, the orbits of low‐flying satellites are gradually lowered by slight atmospheric friction, particularly during periods of maximum solar activity when the upper atmosphere is heated more strongly and balloons outwards.