Geoscience Reference
In-Depth Information
Figure 1.1 A simplified
and schematic
representation of the Earth's
climate system.
ATMOSPHERE
Terrestrial
radiation
Solar
radiation
Atmospheric
gases and aerosols
Clouds
Advection
1
2
Heat
exchange
4
Sea ice
Evaporation
3
Ice sheets
LAND
OCEAN
1 Atmosphere-land coupling
3 Atmosphere-sea ice coupling
4 Atmosphere-ice coupling
2 Atmosphere-biosphere coupling
relationship by the equation F ¼ T 4 where F is the flux of radiation emitted per
square meter, is a constant (5.67 10 8
Wm 2 k 4
in SI units), and T is
an object's surface temperature in degrees Kelvin.
Applying this law, the average temperature at the surface of the Sun is 6000 K.
The average temperature of Earth is 288 K. The temperature at the surface of the
Sun is more than 20 times as high as that of Earth. Twenty raised to the fourth
power is 160 000. Therefore, the Sun emits 160 000 times as much radiation per
unit area as the Earth. The Sun emits radiation in a continuous range of electro-
magnetic waves ranging from long radio waves with wavelengths of 10 5 meters
down to very short waves such as gamma rays, which are less than 10 4
micrometers in length.
Another law of radiant energy (Wien's Law) states that the wavelength of
maximum intensity of radiation is inversely proportional to the absolute tem-
perature. Thus the higher the temperature, the shorter the wavelength at which
maximum radiation intensity occurs. This is given by l max ¼ 2897/T where
T ¼ temperature in degrees Kelvin, and wavelength is in micrometers.
For the Sun, l max is 2897/6000 which equals 0.48 mm. For the Earth l max is given
by 2897/288, a wavelength of 10 mm. Thus the Sun radiates mostly in the visible
portion of the electromagnetic spectrum and the Earth in the infrared (Figure 1.2 ).
There is a thus a fundamental difference between solar and terrestrial radiation and
the ways in which each interacts with the atmosphere and Earth's surface.
Utilization of these laws, and knowledge of Earth-Sun relations, enables the
computation of the amount of energy arriving, the solar constant, and the nature
of solar and terrestrial radiation. These are used to derive budgets of energy
exchanges over the Earth's surface. Box 1.1 provides basic information on this
using the customary symbols.
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