Environmental Engineering Reference
In-Depth Information
4 As substances get hotter, the wavelength at which radiation is emitted will become
shorter (Figure 2.7). This is called Wien's displacement law, which can be represented
as λ
m
= α/T, where λ
m
is the wavelength at which the peak occurs in the spectrum, α is
a constant with a value of 2898 if λ
m
is expressed in micrometres, and
T
is the absolute
temperature of the body.
5 The amount of radiation passing through a particular unit area is inversely proportional
to the square of the distance of that area from the source (1/
d
2
), as shown in Figure
2.8.
EXOGENETIC ENERGY
Now that the principles of radiation have been outlined, we can look at the details of solar
radiation input to Earth in a more meaningful manner. This input is termed
exogenetic
because it originates outside the Earth system.
Because we know the mean distance of Earth from the sun we can work out, from law
5 above, how much radiation Earth should receive. This amount is the solar constant and
has a value of about 1370 W m
−2
at the top of the atmosphere. Recent work from
satellites shows that the solar constant increased about 0·1 per cent between 1986 and
1991, with a decrease from 1992 to 1996 as part of the solar cycle evident in sunspots. A
1 per cent increase would be adequate to cause an increase of 0·5° C in global
temperature. By measuring how much radiation reaches the top of the atmosphere, and
knowing the size of the sun, as well as Earth's mean distance, the emission temperature
of the sun can be determined from law 3. For the photosphere, or visible light surface of
the sun, this value works out to about 6000 K. This figure enables us to determine at what
wavelength most radiation will be emitted from the sun from law 4, that is:
From Figure 2.3 we can see that this value is in the middle of the visible part of the
spectrum. Note that it is the wavelength of blue light.
From the radiation laws it has been possible to determine how much radiation Earth
ought to receive, as well as the amount and properties of solar radiation. Similar
calculations can be made for Earth when we are considering outputs.
The input of energy to Earth at its mean distance from the sun is only an average
value, for changes are taking place all the time. For example, Earth is rotating on its axis
once in twenty-four hours, it is orbiting the sun about once in 365 days and, as its axis of
rotation is at an angle of about 23·5° to the vertical, the distribution of radiation at the top
of the atmosphere is constantly changing. Over even longer periods of time the nature of
Earth's orbit and its angle of tilt also change, thus affecting the amount and distribution
of radiation over Earth. These, however, are important only on a time scale of thousands
of years and will be discussed more fully in Chapter 9.
The sun also emits energy in what is called corpuscular radiation (sometimes referred
to as the
solar wind
), which is composed primarily of ionized particles and magnetic
fields. There is a connection between variations in the strength of the solar wind and
activity on the surface of the sun. This activity is most clearly seen in the form of
