38 / Basics of Environmental Science
Radiant heat and light are both forms of electromagnetic radiation, varying only in their wavelengths,
and the Sun radiates across the whole electromagnetic spectrum. According to Wien’s law^10 , the
wavelength at which a body radiates most intensely is inversely proportional to its temperature, so
the hotter the body the shorter the wavelength at which it radiates most intensely. This is not
surprising, because electromagnetic radiation travels only at the speed of light (beyond the Earth’s
atmosphere, in space, about 300000 km s-1) and the only way its energy can increase is by reducing
the wavelength. Very short-wave (high-energy) gamma (10-4–10-8 μm) and X (10-3–10-5 μm) solar
radiation is absorbed in the upper atmosphere and none reaches the surface. Radiation with a
wavelength between 0.2 and 0.4 μm is called ‘ultraviolet’ (UV); at wavelengths below 0.29 μm,
most UV is absorbed by stratospheric oxygen (O
2
) and ozone (O
3
). The wavelengths between 0.4
and 0.7 μm are what we see as visible light, with violet at the short-wave end of the spectrum and
red at the long-wave end. These are the wavelengths at which the Sun radiates most intensely, with
an intensity peak at around 0.5 μm in the green part of the spectrum. It is the part of the spectrum
to which our eyes are sensitive, for the obvious reason that the most intense radiation is also the
most useful, although some animals have eyes receptive to slightly shorter or longer wavelengths.
Beyond the red end of the visible spectrum lie the infra-red wavelengths (0.7 μm to 1 mm) and,
with increasing wavelengths, microwaves and radio waves, the longest of which have wavelengths
up to about 100 km.
The atmosphere is transparent to wavelengths longer than 0.29 μm, although water vapour absorbs
energy in several narrow bands between 0.9 and 2.1 μm (BARRY AND CHORLEY, 1982, pp. 10
and 15). When radiant energy, as light or heat, strikes the surface of land or water its energy is
absorbed and the surface is warmed. The Earth is not warmed evenly and Figure 2.10 shows how the
energy is distributed. The equator faces the Sun, which is always directly overhead at noon.
Consequently, its radiation is most intense at the equator. With increasing distance from the equator,
the Sun is lower in the sky at all seasons and its radiation covers a larger area less intensely.
Although latitude is obviously important, and places in high latitudes tend to receive less solar
energy than those in low latitudes, cloudiness modifies the general distribution quite strongly. The
equatorial region does not receive the most intense insolation, because for much of the time clouds
shade the surface, reflecting incoming sunlight. Tropical and subtropical deserts, where skies are
mainly clear, receive 50 to 100 per cent more insolation than the equator and the dry interiors of
North America and Eurasia are much sunnier than maritime regions.
Rather less than half of the solar radiation reaching the top of the atmosphere penetrates all the way
to the surface. As Figure 2.11 shows, most of the ‘lost’ incoming radiation is reflected directly back
into space, and about 10 per cent is absorbed or scattered by ozone, water vapour, and particulate
matter in the troposphere.
It is scattering that gives the sky its colour. Radiation bounces off particles (mainly molecules) of
a particular size in relation to its wavelength. All that changes is the direction of the radiation.
There is no loss of energy, but shorter wavelengths scatter more than longer ones. This is called
Rayleigh scattering, after Lord Rayleigh (1842–1919) who discovered it, and it reflects radiation
in all directions. When the Sun is high in a clear sky, violet light is scattered and absorbed very
high in the atmosphere and blue below it. Scattering diffuses the blue light evenly and so the sky
appears blue. If the sky is hazy, dust particles scatter light of all wavelengths and the sky appears
white. When the Sun is low, dust particles scatter light in the orange and red wavelengths, but
shorter wavelengths are absorbed during the much longer passage of the light through the air, and
the sky appears orange or red. Spherical particles larger than those responsible for Rayleigh
scattering (more than about 0.1 μm) scatter light of all wavelengths, mainly without changing its
direction. This is Mie scattering, discovered by Gustav Mie in 1908, and it tends to darken the