The energy that drives our weather systems comes from the sun in the
form of solar radiation. The sun radiates waves of electro-magnetic
and high-energy particles into space. This type of energy can
pass through a vacuum and through gases. The Earth intercepts
the radiation energy
and, as these energy waves pass through the
atmosphere, they are absorbed, scattered and refl ected by gases, air
molecules, small particles and cloud masses (see Figure 2.2).
About a quarter of the total radiation entering the atmosphere reaches the
Earth’s surface directly. Another 18 per cent arrives indirectly after being
scattered (diffused). The surface is warmed as the molecules of rock, soil,
and water at the surface become excited by the incoming radiation; the
energy in the electro-magnetic waves is converted to heat energy as the
surface material absorbs the radiation. A reasonable estimate of energy
can be calculated from the relationship
between radiation and
levels. The amounts received in the British Isles are shown in Figure 2.3
where the differences between winter and summer are illustrated.
|Figure 2.2 Radiation energy reaching the Earth’s surface
showing the proportions that are reflected back and
absorbed as it passes through the atmosphere and that
which reaches plants indirectly. About 5 percent of the
radiation strikes the Earth’s surface but is reflected back
(this is considerably more if the surface is light coloured,
e.g. snow, and as the angle of incidence is increased).
|Figure 2.4 Effect of angle of incidence on heating
at the Earth’s surface. A higher proportion of the
incoming radiation is reflected as the angle of
incidence increases. Note also that a higher
proportion of the incoming radiation is absorbed
or reflected back as it travels longer through
the atmosphere in the higher latitudes.
However, the nature of the surface has a signifi cant effect on the
proportion of the incoming radiation that is absorbed. The sea can absorb over 90 per cent of radiation when the sun is overhead, whereas
for land it is generally between 60 and 90 per cent. Across the Earth
darker areas tend to absorb more energy than lighter ones; dark soils
warm up more quickly than light ones; afforested areas more than
lighter, bare areas with grass are between these values. Where the
surface is white (ice or snow) nearly all the radiation is reflected.
Effect of latitude
Over the Earth’s surface some areas become warmed more than others
because of the differences in the quantity of radiation absorbed. Most
energy is received around the Equator where the sun is directly overhead
and the radiation hits the surface at a right angle. In higher latitudes such
as the British Isles more of the radiation is lost as it travels further through
the atmosphere. Furthermore, the energy waves strikes the ground at an
acute angle, leading to a high proportion being refl ected before affecting
the molecules at the surface (see Figure 2.4).
|Figure 2.3 Radiation received in the British Isles ; mean daily radiation given in megajoules per metre square. (a)
January (b) July.
As a consequence of the above, more energy is received than lost over
the span of a year in the region either side of the Equator between the
Tropic of Capricorn and Tropic of Cancer. In contrast, to the north
and south of these areas more energy radiates out into space, which
would lead to all parts of this region becoming very cold. However, air
and water (making up the Earth’s atmosphere and oceans) are able to
redistribute the heat.
Movement of heat and weather systems
Heat energy moves from warmer areas (i.e. those at a higher temperature)
into cooler areas (i.e. those at a lower temperature) and there are three
types of energy movement involved. Radiation
energy moves efficiently through air (or a vacuum), but not through water
or solids. Heat is transferred from the Earth’s
surface to the lower layers by conduction
. As soil
surfaces warm up in the spring, temperatures in
the lower layers lag behind, but this is reversed
in the autumn as the surface cools and heat is
conducted upwards from the warmer lower layers.
At about one metre down the soil temperature
tends to be the same all the year round (about
10°C in lowland Britain).
Heat generated at the Earth’s surface is also
available for redistribution into the atmosphere.
However, air is a poor conductor of heat (which
explains its usefulness in materials used for
insulation such as polystyrene foam, glass fi bre
and wool). It means that, initially, only the air immediately in contact
with the warmed surface gains energy. Although the warming of the
air layers above would occur only very slowly by conduction, it is the
process of convection
that warms the atmosphere above. As fl uids are
warmed they expand, take up more room and become lighter. Warmed
air at the surface becomes less dense than that above, so air begins to
circulate with the lighter air rising, and the cooler denser air falling to
take its place; just as with a convector heater warming up a room. This
circulation of air is referred to as wind
In contrast, the water in seas and lakes is warmed at the surface making
it less dense which tends to keep it near the surface. The lower layers
gain heat very slowly by conduction and generally depend on gaining
heat from the surface by turbulence. Large-scale water currents
created by the effect of tides and the winds blowing over them.
On a global scale, the differences in temperature at the Earth’s surface
lead to our major weather systems
. Convection currents occur across
the world in response to the position of the hotter and colder areas and
the influence of the Earth’s spin (the Coriolis Effect). These global air
movements, known as the trade winds, set in motion the sea currents,
follow the same path but are modifi ed as they are defl ected by the
continental land masses (see Figure 2.5).
|Figure 2.5 Global sea and wind movements . Warmer and colder water currents set in motion by the wind
circulation around the world.