Highest Layers

There is no air beyond 32,000 km because the Earth's gravitational attraction there is exceeded

Figure 1.10 Typical features of the layers and zones of the atmosphere at various heights.

by the centrifugal force away, caused by the Earth's spinning. Then, down to about 200 km from the ground, there is the thermosphere, so-called because of high temperatures, nominally around 1,000 K, due to absorbing the energy of ultra-violet radiation (Section 1.4). Such a temperature of what is almost a perfect vacuum merely indicates the high speed of the individual molecules present, energised by solar radiation.

The thermosphere corresponds very approximately to the heterosphere, the region above 100 km where the air is stratified, with lighter gases above heavier. At about 100 km is the turbopause, bounding the homosphere beneath, where the air is mixed enough to have a uniform chemical composition.

Within the homosphere are three layers (or 'spheres'), each capped by a transition zone (or 'pause'). The mesopause separates the thermosphere from the mesosphere (the 'middle region') beneath, and may lie at about 80 km elevation. Temperatures there are typically as low as -90°C. There is normally very little moisture in the mesosphere, though thin purplish patches of 'nacreous' (i.e. pearl-like) cloud occasionally occur at 60-80 km elevation (Chapter 8). Lapse conditions exist in the mesosphere, but less steeply than in the troposphere (Figure 1.10).

The next dividing zone is the stratopause at around 50 km, which separates the mesosphere above from the stratosphere below. More than 99.7 per cent of the atmosphere is found below the stratopause. The temperature there is about 0°C on account of heat produced by (i) the absorption of solar UV radiation by ozone, formed after the dissociation of some of the oxygen and then recombination as ozone (Section 1.4), and (ii) absorption by the ozone of longwave radiation from the ground beneath (Chapter 2).

There is normally a negative lapse rate throughout the stratosphere, due to the relatively high temperature at the stratopause. But the stratosphere has a slightly positive lapse rate in the polar winter (Figure 1.9), due to the absence of sunshine for several months. The result is that temperatures are below -100°C at the polar stratopause in winter, and this produces upper winds exceeding 60 m/s at the highest latitudes (Chapter 12). On the other hand, the temperature at the polar stratopause often exceeds 30°C in summer because of the long days then.

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