Geoscience Reference
In-Depth Information
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 2.14
).
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.
380
370
360
350
340
330
320
310
300
290
280
270
1700
1800
1900
2000
Year
Measuring radiation from space
NEW DEVELOPMENT
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
s
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 it has to be at a height of about 35,000 km above the equator, effectively rotating at the same rate as Earth.
Geostationary satellites 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 50º 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 μm 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 instrument screen. Second, there are differences between satellite systems
in terms of their spectral responses and instrumental calibration. Third, there are problems in making generalizations
about Earth as a whole when assumptions have to be made about areas that are less well monitored or when temporal
sampling is not systematic. For example, the polar orbiting satellites will cover different parts of Earth's surface at
different times of the day, depending upon their orbit. This can be significant in the diurnal variability of cloud. Hence
many of the figures in this chapter are not based on the 2000s, as such studies are time-consuming and difficult.
Nevertheless we now know much more about the magnitude of inputs and outputs of energy within the
Earth-atmosphere system.
