Geoscience Reference
In-Depth Information
of water vapour are carried upward by high-level
convection caused by the vertical decrease of tempera-
ture in the mesosphere. However, their formation may
also be related to the production of water vapour through
the oxidation of atmospheric methane, since apparently
they were not observed prior to the Industrial Revolu-
tion. The layers between the tropopause and the lower
thermosphere are commonly referred to as the
middle
atmosphere
, with the upper atmosphere designating the
regions above about 100 km altitude.
5 Exosphere and magnetosphere
The base of the exosphere is between about 500 km and
750 km. Here atoms of oxygen, hydrogen and helium
(about 1 per cent of which are ionized) form the tenuous
atmosphere, and the gas laws (see B, this chapter)
cease to be valid. Neutral helium and hydrogen atoms,
which have low atomic weights, can escape into space
since the chance of molecular collisions deflecting
them downward becomes less with increasing height.
Hydrogen is replaced by the breakdown of water vapour
and methane (CH
4
) near the mesopause, while helium
is produced by the action of cosmic radiation on
nitrogen and from the slow but steady breakdown of
radioactive elements in the earth's crust.
Ionized particles increase in frequency through the
exosphere and, beyond about 200 km, in the magneto-
sphere there are only electrons (negative) and protons
(positive) derived from the solar wind - which is a
plasma of electrically conducting gas.
4 Thermosphere
Atmospheric densities are extremely low above the
mesopause, although the tenuous atmosphere still
effects drag on space vehicles above 250 km. The lower
portion of the thermosphere is composed mainly of
nitrogen (N
2
) and oxygen in molecular (O
2
) and atomic
(O) forms, whereas above 200 km atomic oxygen pre-
dominates over nitrogen (N
2
and N). Temperatures
rise with height, owing to the absorption of extreme
ultraviolet radiation (0.125 to 0.205 µm) by molecular
and atomic oxygen, probably approaching 800 to 1200
K at 350 km, but these temperatures are essentially
theoretical. For example, artificial satellites do not
acquire such temperatures because of the rarefied air.
'Temperatures' in the upper thermosphere and exos-
phere undergo wide diurnal and seasonal variations.
They are higher by day and are also higher during a
sunspot maximum, although the changes are only repre-
sented in varying velocities of the sparse air molecules.
Above 100 km, cosmic radiation, solar X-rays and
ultraviolet radiation increasingly affect the atmosphere,
which cause
ionization
, or electrical charging, by sepa-
rating negatively charged electrons from neutral oxygen
atoms and nitrogen molecules, leaving the atom or
molecule with a net positive charge (an
ion
). The term
ionosphere
is commonly applied to the layers above
80 km. The Aurora Borealis and Aurora Australis are
produced by the penetration of ionizing particles
through the atmosphere from about 300 km to 80 km,
particularly in zones about 10 to 20° latitude from the
earth's magnetic poles. On occasion, however, aurora
may appear at heights up to 1000 km, demonstrating the
immense extension of a rarefied atmosphere.
SUMMARY
The atmosphere is a mixture of gases with constant
proportions up to 80 km or more. The exceptions are
ozone, which is concentrated in the lower strato-
sphere, and water vapour in the lower troposphere.
The principal greenhouse gas is water vapour. Carbon
dioxide, methane and other trace gases have increased
since the Industrial Revolution, especially in the
twentieth century due to the combustion of fossil fuels,
industrial processes and other anthropogenic effects,
but larger natural fluctuations occurred during the
geologic past.
Reactive gases include nitrogen and sulphur and
chlorine species. These play important roles in acid
precipitation and ozone destruction. Acid precipitation
(by wet or dry deposition) results from the reaction
of cloud droplets with emissions of SO
2
and NO
x
.
There are large geographical variations in acid
deposition. The processes leading to destruction
of stratospheric ozone are complex, but the roles of
nitrogen oxides and chlorine radicals are very important
in causing polar ozone holes. Aerosols in the atmos-
phere originate from natural and anthropogenic
sources and they play an important but complex role
in climate.
