Geoscience Reference
In-Depth Information
5
Terrestrial Radiation
Introduction
Surface energy transfer as electromagnetic radiation is a very substantial
component of the Earth
atmosphere system; it is important because it is the driving
force for hydroclimatic movement. In comparison with other energy exchanges that
are slower because they involve transfers via the physical movement of molecules
or portions of air, radiation transfer occurs at the speed of light,
c
−
10
8
m s
−1
and is effectively instantaneous. As Fig. 5.1 shows, electromagnetic radiation
covers a wide spectrum of wavelengths. Note that in physics it is conventional to
use the symbol
l
to describe wavelength and this convention is adopted in this
chapter, although elsewhere in this text the symbol
l
is used to describe the latent
heat of vaporization of water.
When considering the terrestrial radiation balance at the surface, it is helpful to
remember that anything with a temperature above absolute zero emits radiation
with a spectrum and at a rate that reflects the temperature of the emitting entity.
In the case of terrestrial radiation two radiators are important, the Sun, and the
Earth's surface and atmosphere. The Sun has a temperature of around 6000 K and
emits shortwave or
solar radiation
, while the Earth's surface and lower atmosphere
typically has a temperature of
=
3
×
290 K and emits thermal or
longwave radiation
.
Consequently, at the Earth's surface the majority of the radiation exchange is via
radiation which lies in the wavelength range 0.1-100
∼
μ
m (1
μ
m = 10
−6
m). Most
is in the
visible
(0.39-0.77
μ
m),
near infrared
(0.77-25
μ
m) and
far infrared
(25-1000
m)
waveband. The visible portion of the spectrum is the radiation we see, but we are
aware of infrared radiation because it warms us, and ultraviolet radiation because
it tans our skin.
Figure 5.2 shows that in practice most solar energy is in the wavelength range
0.15-4
μ
m) wavebands, but there is also some in the ultraviolet (0.001-0.39
μ
μ
m while most of the energy in terrestrial radiation is in the wavelength
