Earth Sciences / 53The lowest layer, the troposphere, extends from the surface to an upper boundary, the tropopause,
the height of which varies, but averages about 16 km at the equator and 8 km at the poles.
Within the troposphere temperature decreases with height by an average of about 6.5°C km-1
(called the ‘lapse rate’). Above the tropopause temperature remains constant with height to
about 20 km. Rising tropospheric air is trapped below the region of constant temperature, which
forms a permanent temperature inversion, a layer in which temperature remains constant or
increases with altitude, rather than falling. It is this inversion which confines meteorological
phenomena to the troposphere.
Temperature then increases from a minimum of about -80°C at the equator in summer, when the
tropopause is at its highest, to 0°C or even higher at about 50 km. This region is the stratosphere and
its upper boundary is the stratopause. In the mesosphere, above the stratopause, temperature once
more decreases with height, to about -90°C at the mesopause, about 80 km, then rises again through
the thermosphere. At about 350 km the temperature may exceed 900 °C, probably because of the
energy imparted by absorption of ultraviolet radiation by atomic oxygen, but the air is so rarefied
that objects such as satellites are not warmed by it, although it still exerts measurable drag on spacecraft
moving through it.
Between about 30 and 60 km the density of oxygen molecules is high enough to intercept most
of the incoming solar ultraviolet radiation at wavelengths below 0.29 μm. The energy imparted
to them separates the molecules (O 2 → O + O ). Some of the oxygen atoms then combine with
oxygen molecules to form ozone (O + O 2 → O 3 ). Ozone is unstable and may decompose either
by encountering more oxygen atoms (O 3 + O → 2O 2 ) or by absorbing more ultraviolet radiation.
Ozone is, therefore, constantly forming, decomposing, and re-forming, and the process is in
equilibrium above about 40 km. There is also some transport of ozone from low to high latitudes.
There is some mixing of stratospheric air, however, as a result of which a small amount of ozone
is transported downward, to accumulate between about 20 and 25 km. This is the ‘ozone layer’.
Its density varies, being lowest over the equator and high over latitudes above 50°. Ordinarily,
ozone levels are also high over polar regions in early spring. This is because ozone is neither
formed nor destroyed during the polar night, when there is no radiation to drive the reactions,
and ozone transported from lower latitudes is stored (BARRY AND CHORLEY, 1982, pp. 2–3).
Despite being known as the ‘ozone layer’, if the air at that altitude were compressed to sea-level
pressure the ozone would contribute only about 3 mm to it. In itself, the ozone layer does not
shield the surface from ultraviolet radiation, but indicates that radiation is being absorbed at a
greater height, shielding both the surface and the ozone layer.
The thickness of the ozone layer is often reported in Dobson units (DU). This unit was devised by
G.M.B.Dobson, a British physicist who studied stratospheric ozone in the 1920s. It refers to the
thickness of the layer that a gas would form if all the other atmospheric gases were removed and the
gas in question were subjected to standard sea-level pressure. In the case of ozone, 1 Dobson unit
corresponds to a thickness of 0.01 mm and the amount of ozone in the ozone layer is typically 220–
460 DU, corresponding to a layer 2.2–4.6 mm thick.
The depletion of the ozone layer over Antarctica, first observed in 1986 (ALLABY, 1992, pp.
159–161) by a British scientist, occurs just as spring is commencing. During the polar night,
a vortex of very still air forms over Antarctica within which the temperature may be as low as
-84°C. Clouds of ice crystals, called polar stratospheric clouds (PSC), form inside the vortex
and a series of chemical reactions on the surface of the ice crystals results in chlorine monoxide
(ClO) combining to form Cl 2 O 2. These molecules break down when exposed to sunlight (Cl 2 O 2
→ 2Cl + O 2 ) and the chlorine atoms combine with ozone in two steps which release free
chlorine once more to repeat the process so that a single chlorine atom can destroy many