FIGURE 3.1. Vertical temperature profile for the ''US standard atmosphere" at 40° N in December.

3.1.2. Atmospheric layers

Coming down from the top of the atmosphere, the first hot spot evident in Fig. 3.1 is the thermosphere, where the temperature is very high and variable. It is here that very short wavelength UV is absorbed by oxygen (cf. Section 2.2), thus heating the region. Molecules (including O2 as well as CO2, the dominant IR emitter at this altitude) are dissociated (photolyzed) by high-energy UV (A < 0.1 ^m). Therefore, because of the scarcity of polyatomic molecules, IR loss of energy is weak, so the temperature of the region gets very high (as high as 1000 K). The air is so tenuous that assumptions of local thermodynamic equilibrium, as in blackbody radiation, are not applicable. At and above these altitudes, the atmosphere becomes ionized (the ionosphere), causing reflection of radio waves, a property of the upper atmosphere that is of great practical importance.

Below the mesopause at about 80-90 km altitude, temperature increases, moving down through the mesosphere to reach a maximum at the stratopause, near 50 km, the second hot spot. This maximum is a direct result of absorption of medium wavelength UV (0.1 nm to 0.35 ^m) by ozone. It is interesting to note that ozone concentration peaks much lower down in the atmosphere, at heights of 20-30 km, as illustrated in Fig. 3.2. This is because the ozone layer is very opaque to UV (cf. Fig. 2.6), so most of the UV flux is absorbed in the upper parts of the layer, and there is little left to be absorbed at lower altitudes.

The reason for the existence of ozone at these levels is that it is produced here, as a by-product of the photodissociation (photolysis) of molecular oxygen, producing atomic oxygen, which may then combine with molecular oxygen thus:

where hv is the energy of incoming photons (v is their frequency and h is Planck's

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