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.
