Environmental Engineering Reference
In-Depth Information
Hydrogen
H
2
0·00005
Xenon
Xe
0·00009
SHORT-WAVE RADIATION IN THE ATMOSPHERE
As our beam of sunlight enters the atmosphere it first passes through the thermosphere
and the mesosphere with little change. In the stratosphere the density of atmospheric
gases increases. There is more oxygen available which reacts with the shortest or ultra-
violet wavelengths and effectively removes them, warming the atmosphere and
producing ozone in the process (Figure 3.1). About 3 per cent of the original beam is
converted to heat at this stage (Figure 3.3).
As we descend into the troposphere the atmosphere becomes rapidly denser and so
there is greater interaction between the sunlight and the atmospheric gases. The size of
the gas molecules of the air is such that they interact with the insolation, causing some of
it to be scattered in many directions. This process depends on wavelength. The shorter
waves are scattered more than the longer waves and so we see these scattered waves as
blue sky. If the reverse were true the sky would be permanently red, and if there were no
atmosphere, as on the Moon, the sky would be black. Dust and haze in the atmosphere
produce further scattering, but not all of this is lost. Some of the scattered radiation is
returned to space, but much is directed downwards towards the surface as diffuse
radiation. This is the type of radiation which we also experience during cloudy conditions
with no direct sunlight when the solar beam is 'diffused' by the water droplets or ice
particles of the clouds.
Another type of short-wave energy loss is absorption. Some gases in the atmosphere
absorb certain wavelengths (Figure 2.9), as do clouds, dust and haze The short-wave
radiation is converted to long-wave radiation. In this way we have a warming of the
atmosphere, though the amounts involved are small. The most important loss of short-
wave radiation in its path through the atmosphere is by reflection. The water droplets or
ice crystals in clouds are very effective in reflecting insolation. Satellite evidence shows
that, for Earth, a mean figure of 19 per cent of the original insolation is reflected by
clouds. The lowest and thickest clouds tend to reflect most, while the thin, highlevel ice
clouds have an albedo of only about 30 per cent.
By now, the beam has reached the ground surface with, as a global average, about 50
per cent of its original energy. Even then, not all of it is absorbed, as the surface itself has
an albedo. The global average albedo represents some 6 per cent of the radiation at the
top of the atmosphere, so the loss is not great. However, the figure may seem large when
expressed as a percentage of the radiation actually reaching the surface. For example, the
albedo of freshly fallen snow may reach as high as 90 per cent (Table 3.2). The greatest
variability is over water. When the sun is high in the sky, water has a very low albedo.
That is why oceans appear dark on satellite photographs (Plate 3.1). At low angles of the
sun, as at dawn or in midwinter in temperate and sub-polar latitudes, the albedo may
reach nearly 80 per cent.
The sunlight reaching Earth's surface which is not reflected by Earth is absorbed and
converted into heat energy. The distribution of energy received at the surface is shown in
