Geoscience Reference
In-Depth Information
the surface still retains some of the heat gained during the
summer. The air, therefore, remains relatively warm
compared with spring even through the sun is at the same
midday zenith angle.
The seasonal pattern of radiation and associated
temperature conditions varies latitudinally. In polar areas
the sun never gets high in the sky, but the length of the
day varies markedly, so that during summer months these
areas experience perpetual daylight. Conversely, in the
winter months they are in continuous darkness. The
seasonal radiation balance is therefore very variable. At the
north pole, for example, from April to September there is
a potential continuous radiation surplus, for the sun
would shine for twenty-four hours per day if the sky was
cloud-free, so night-time cooling is less. In contrast, for
the rest of the year a radiation deficit occurs. No insolation
is experienced for six months, so radiational cooling
continues, interrupted only by the transfer of air from
warmer latitudes.
The pattern in the tropics is very different. Here the sun
never strays far from its overhead position; seasonal
variations in radiation are limited and the diurnal
variation becomes dominant.
The pattern of energy input to Earth's surface as
shown in
Figure 3.3
is a vital element in determining the
thermal regime. As we shall see, it is not the only factor
involved and the actual surface temperatures (
Figure
40
Desert
site
30
20
Oceanic
site
10
0
0
6
12
18
24
Hours
ranges. Oceanic sites have a small diurnal temperature range,
whereas in a desert the range is high.
of insolation received by Earth and, therefore, the degree
of atmospheric heating. Considering again our area in
London, we would find that in winter the maximum
elevation of the sun, at midday, was about 16
, for the sun
stands approximately over the Tropic of Capricorn. Thus
the rays of the sun still strike the surface at a relatively low
angle and the degree of midday heating is limited.
As the sun moves northwards to the equator and thence
to the Tropic of Cancer its midday position rises and
the rays strike the surface less obliquely. Moreover, the
days become longer and the nights shorter. Maximum
temperatures increase until, about July, they reach their
highest values, slightly after the maximum radiation in late
June. From then until mid-December the sun returns
south, its midday position in the sky declines, the quantity
of insolation received at the surface is reduced and so
temperatures fall.
It is apparent that, in London, the winter months
represent a period when incoming radiation is low.
Outputs of energy from Earth continue, however, so the
area experiences a net radiation deficit. During the spring,
as the overhead sun moves north of the equator, radia-
tion inputs rise to match outputs, but the degree of
atmospheric warming is restricted because much of the
excess energy is used to reheat the ground and the oceans.
By August the ground has warmed up; during autumn the
sun returns to its position over the equator but now
CONCLUSION
In this chapter we have shown how energy is transmitted
through the atmosphere from space and from Earth's
surface. The different response of the atmosphere to
long- and short-wave radiation forms the basis of the
greenhouse effect. The spatially and temporally varying
inputs and outputs of radiant energy from the surface
form the energy gradient between surface and atmos-
phere and between tropics and polar regions. From this
we find that the atmosphere will always have a radiation
deficit which requires energy transfer in the form of
latent and sensible heat, and there is another radiant
energy gradient between a tropical surplus and a polar
deficit. This second gradient forms the driving force for
the atmospheric circulation whereby heat has to be
transferred polewards to offset the radiational deficit. It
is this energy exchange which forms the basis of our
climatic system.
