Environmental Engineering Reference
In-Depth Information
Figure 2.9
Absorptivity at different wavelengths (µm) by
selected constituents of the atmosphere and by the
atmosphere as a whole.
Source: After Fleagle and Businger (1963).
the larger particles of dust in the lower atmosphere. The result is that more of the red
wavelength is scattered, producing more colourful skies at sunrise and sunset.
Absorption
of the radiant energy has more far-reaching consequences than reflection
or scattering. As an object absorbs energy its temperature rises, because the radiant
energy is converted to heat (thermal energy). Reradiation of this energy tends to occur at
a temperature different from that of the initial, radiating object, and thus the radiation
emitted is at a different wavelength. Earth, for example, is considerably cooler than the
sun; thus the energy it emits is characteristically of longer wavelengths than the initial
solar inputs.
We can summarize the radiation laws as follows:
1 All substances emit radiation when their temperature is above absolute zero (−273° C
or 0 K).
2 Some substances absorb and emit radiation at certain wavelengths only. This is true
mainly of gases.
3 If the substance is an ideal emitter (a black body) the amount of radiation given off is
proportional to the fourth power of its absolute temperature. This is known as the
Stefan - Boltzmann law and can be represented as
E
= σ
T
4
, where
E
equals the
maximum rate of radiation emitted by each square centimetre of the surface of the
object, σ is a constant (the Stefan - Boltzmann constant) with a value of 5·67 × 10−
8
W m−
2
K
−4
, and
T
is the absolute temperature.
