Geoscience Reference
In-Depth Information
Figure 2.6
Energy transfers and
transformations.
Solar
energy
Radiation
to space
Re-radiated
and reflected
to space
Reflected and re-radiated
to space
Heat
Absorbed
by
atmosphere
Turbulence,
friction
Radiation,
evaporation,
convection,
conduction,
condensation
exchanges
K.E. in the form
of air circulation
(convection induced
by unequal
distribution of heat)
Absorbed
by surface
rises, the radiant energy increases in intensity, but its
wavelength decreases; as the temperature falls the intensity
decreases and the wavelength increases (
Figure 2.7
).
In
addition, the amount of radiation reaching any object
is inversely proportional to the square of the distance
from the source (
Figure 2.8
).
This distance decay factor
accounts for the difference in solar inputs to the various
planets in our solar system.
To a certain extent radiation is able to penetrate matter,
as, for example, x-rays, which can pass through the human
body, but most radiant energy is either absorbed or
reflected by objects in its path. Absorption occurs when
the electromagnetic waves penetrate but do not pass
through the object; reflection involves the diversion or
deflection of the waves from the surface of objects without
any change of wavelength. The ability of an object to
absorb or reflect radiant energy depends upon a number
of factors, including the detailed physical structure of the
material, its colour and surface roughness, the angle of
the incident radiation and the wavelength of the radiant
energy.
An object that is able to absorb all the incoming
radiation is referred to as a
black body
. Although it has
conceptual value, a perfect black body does not exist in
reality. All objects absorb a proportion of incoming energy
and reflect the remainder. The amount of radiation
reflected from a surface is called the
albedo
. The term is
most frequently used for the visible part of the spectrum.
It is calculated by dividing the amount reflected by the
total amount arriving at a surface and is often expressed
as a percentage. The colour of the surface determines the
amount reflected. Solar collection panels are matt black
to ensure that the maximum amount of short-wave
energy is absorbed and converted to heat. Differences also
occur according to the wavelength of the energy. Thus
snow and sand both absorb
long-wave radiation
(5-50
μm) quite efficiently, but they reflect relatively large
proportions of
short-wave radiation
(0·4-0·8 μm).
Indeed, under constant conditions, it is possible to define
the wavelengths that specific materials selectively absorb
and emit, and this knowledge can be used to characterize
or identify materials through remote sensing. It is
frequently used in astronomy to determine the gases
present in stars.
Whereas solid substances usually absorb most
wavelengths of radiation, gases tend to be very selective
