Environmental Engineering Reference
In-Depth Information
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,
radiation 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 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 3.2 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 3.14) show many differences from the pattern
shown in the figure.
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 atmosphere 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.
NOTE
1 The specific heat of a substance is the amount of heat required to raise the temperature of 1 g
of that substance by 1°C. This is defined at a constant pressure because adding heat normally
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