Environmental Engineering Reference
In-Depth Information
radiation. When the sun is low in the sky, the steeper slopes facing the sun will receive
the highest values. As Earth is a sphere at a great distance from the sun, the sun's rays
appear parallel and hit the surface at different angles (Figure 2.13).
A secondary effect which further decreases radiation intensity is the longer path
through the atmosphere at higher latitudes. Scattering and absorption will be higher,
though they increase diffuse radiation at the expense of direct radiation. The amount of
scattering and absorption will vary, depending upon the degree of haziness of the
atmosphere. Where the atmosphere is very dusty, as in semi-arid or desert areas, more
radiation will be absorbed and scattered, preventing it from reaching the ground surface.
As the dust particles are much larger than gas molecules, scattering is not dependent upon
wavelength and the sky has a whitish hue rather than the deep blue of a clear atmosphere.
This effect is also noticeable over urban areas, where pollution produces the same effect.
LATITUDINAL RADIATION BALANCE
To see how much radiant energy we have available at any location we must know how
much radiation is being lost as well as how much is reaching that location. Long-wave
radiation emission is proportional to the absolute temperature of the surface. It is far less
variable than the input
MEASURING RADIATION FROM SPACE
new developments
Until the appearance of artificial satellites we could not directly measure the components
of Earth's radiation budget. Estimates were made of solar input and the proportions of
reflected short-wave and emitted long-wave radiation but they were based on a variety of
assumptions. Now radiation measurements can be taken from satellites with one of two
types of orbit. Satellites can follow a polar orbit at a height of between 500 and 1,500 km
above Earth's surface. They cross the equator at about 90° and take about ninety minutes
to complete the full orbit, obtaining information from both the sunlit and dark sides of the
globe. They give good resolution but their field of view changes from one orbit to the
next.
The other type of satellite is known as geostationary as it appears to hold a fixed
position above the surface. To do this they have to be at a height of about 35,000 km
above the equator, effectively rotating at the same rate as Earth. They therefore
continuously view the same section of Earth. Because of their altitude they have a poorer
resolution and, because of Earth's curvature, information polewards of about 50° is more
limited. They do give continuous information for the field of view.
The satellites contain sensors which can measure particular spectral wavelengths.
Short-wave sensors can pick up solar input and Earth's albedo from reflected radiation
between 0·4 and 1 µm. Long-wave sensors can measure long-wave emission from the
surface, clouds and the atmosphere. Difficulties arise from a variety of sources. First, it is
not easy to compare the results obtained from the satellites with those obtained at the
ground surface. In some cases this is because the methods are not measuring precisely the
same things e g surface emission temperatures and shade air temperature recorded in an
