The composition of the atmosphere
A mixture of gases surrounds the earth; gases which are
in ceaseless motion, but
which are retained by the force of the earth's gravity. This is the atmosphere,
densest at sea-level, with all but 3 per cent, of its mass within 30 kilometres of the
surface, and about half of it below 6 000 m.
Dry surface air consists mainly
of nitrogen, about 78 per cent., and oxygen, 21 per
cent., by volume. Among the smaller quantities of other gases, carbon dioxide,
some 0.3 per cent., and the varying amounts of water vapour in moist air, are very
important indeed as far as climates, and animal and vegetable life are concerned.
Variable amounts of tiny particles, such as dust, smoke, and salt crystals may have
high local concentrations and affect weather in a number of ways. At high levels
ozone, another form of oxygen, is present in small but important quantities.
We are principally concerned with, and
directly affected by, the lower, denser part
of the atmosphere, known as the troposphere. In this air there is usually a fairly
rapid fall in temperature with altitude, until an overlying layer of relatively warm air
causes an abrupt change, at a height of about 8 000 m near the poles and 17 000
m in the tropics; this is known as the tropopause. The actual height of the
tropopause varies also with the season and weather conditions. Immediately above
this, temperatures cease to decrease with altitude, and there may be a slight
increase in temperature. In general, the temperature between 12-22 km remains at
about -50° C.
This upper part of the atmosphere, known
as the stratosphere, is dust-free and
cloudless, and above the height reached by convectional movements of the
troposphere. However, there are marked temperature differences between parts of
the stratosphere in higher latitudes and those in lower latitudes. The consequent
variations in the density of the upper air results in strong meridional air movements
at great heights. The ozone concentration at this height absorbs radiation from the
sun. This concentration is greatest over polar regions where, as a result, the ozone
layers of the stratosphere tend to be particularly warm in summer. In winter, the lack
of insolation results in particularly cold upper air over the polar regions. These
seasonal contrasts between upper air in the higher and lower latitudes means that
in parts of the stratosphere there are strong horizontal winds.
But our familiar weather phenomena, including
the massive towering thunderclouds,
are confined to the troposphere, even though their causes may not be. So to
provide a background to a study of climatic conditions we turn first to the
troposphere, where vertical convection currents do disturb the atmosphere, and
masses of air flow horizontally from one latitude to another, as "advection" currents,
taking with them their contents of moisture and heat energy.
Air movements and the transfer of heat energy
The earth and its atmosphere receive heat energy from the sun (solar insolation).
Some is returned by scattering and reflection, and much is lost by their own
radiation to outer space. An energy balance has been established, which, as far as
present climatic conditions are concerned, is being maintained; so that the earth
and the atmosphere taken as a whole are becoming neither hotter nor colder.
Short-wave radiation from the sun
passes through the atmospheric gases fairly
effectively. It is not absorbed by most of them; though at high altitudes ozone is a
good absorber, and at low levels carbon dioxide and water vapour absorb a certain
amount. Yet, in fact, only about half this short-wave radiation is absorbed by the
earth's surface; for much is reflected, by cloud surfaces and by the earth itself, and
tiny particles, such as dust, scatter it, so that some is lost into space. The loss by
scattering and reflection, which averages something like two- fifths of all the
incoming short- wave radiation, is sometimes called the albedo of the earth. On a
local basis the term "albedo" is used to express the ability of a surface to reflect
insolation. Fresh fallen snow has an albedo of some 85 per cent., while for various
forms of vegetation it ranges from 10-25 per cent.
Though the atmosphere absorbs little energy
direct from incoming radiation, it
receives much from the earth; and is thus mainly heated from below. As the surface
of the earth absorbs energy its temperature increases. It, too, radiates energy,
though in this case with a long wavelength, which can be strongly absorbed by the
atmosphere. The water vapour in the air and water droplets, and hence clouds, take
up a great deal of this energy emitted by the earth. As the atmosphere absorbs
energy, its own temperature is raised, and it too radiates heat, some downwards to
the earth and some outwards to be lost in space.
The overall picture, therefore, is of
an atmosphere relatively transparent to short-
wave radiation but gaining energy rather from long-wave radiation; resulting in the
maintenance of fairly high temperatures at and near the surface—often known, for
obvious reasons, as a "greenhouse effect".
So far, this is a generalised picture
about the earth as a whole: but though there is
an overall balance between heat energy received and lost into space this does not
mean that there are uniform conditions in the troposphere. The insolation received
varies, of course, with latitude. In low latitudes, where the midday sun is high in the
sky and insolation is strong, the daily amount of incoming radiant energy exceeds
the outgoing; but in the middle and high latitudes, where the sun's rays are oblique
to the surface and the insolation is less intense, more energy is lost from the earth
than is received. If, therefore, there is an overall heat balance, and if the low
latitudes are not to become hotter and hotter, and the high latitudes colder and
colder, some of the heat energy must be transferred horizontally, by advection, from
lower to higher latitudes.
The unequal heating of the earth's surface
is not, however, simply a matter of
gradual change from low to high latitudes. The location of the hottest parts of the
earth's surface vary according to the seasons; land and water masses absorb and
radiate heat at different rates; and variations in the topography and texture of the
surface affect its own temperature and that of the air above. Nor does air circulation
only depend on such temperature differences. The physical make-up of the
atmosphere varies from place to place: masses of air acquire different properties
as they circulate, moving both vertically and horizontally, and changing their density
and water content. The direction of rotation of the earth also affects the pattern of
air circulation.
We cannot hope to appreciate all the causes
nor investigate all the influences which
are responsible for the complex and changing atmospheric movements;
meteorologists are only at the beginning of their investigations into the mechanisms
of such circulations. However, it is possible to observe that there are types of
atmospheric movement which occur, and recur, in place and time with some
regularity; and it is possible to map average conditions, and to investigate the
properties of the air which gives rise to these conditions. We may thus build up a
picture of the distribution of various types of climate, and can then investigate the
more changeable features of those climates. |
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