Table 1: Main constituents of the earth's atmosphere
* a concentration near the earth's surface
Nitrogen, oxygen and argon account for about 99.99% of the permanent
gases. Of the variable constituents, carbon dioxide can be somewhat variable
in concentration on a localised basis at low levels. Water vapour content
may vary from about 0 to 4%, and ozone concentrations also vary markedly.
In addition to these variable constituents there are also aerosols and hydrometeors
which can vary widely in space and time.
Atmospheric gases critically affect the earth's global energy balance through
absorption and re-emission and through the role they play in global geophysical
cycles. Solar radiation reaching the earth's surface is determined by atmospheric
gases. For example harmful UV radiation is blocked by the ozone layer. Also,
"windows" in which atmospheric effects are minimal allow ground-based
measurements of celestial objects, and satellite-based measurements of the
earth's surface or clouds for remote sensing applications.
Table 2 gives the main atmospheric gases which absorb radiation along with
their absorbing regions, at ultraviolet (UV), visible, infrared (IR) and
microwave wavelengths. The main spectral regions ("windows") for
which atmospheric absorption is small, are listed at the bottom of the table.
Table 2: From Smith (1985), p391
Atmospheric absorption features are shown in Figure1 for ground level
and at an altitude of 11 km. Regions of minimal absorption (windows) are
the troughs in the curves.
Figure 1: (a) Atmospheric absorption at ground level for diffuse
terrestrial radiation and for solar radiation, with a zenith angle of 50
degrees. (b) Same as in a but for the layer of atmosphere above 11km. After
Wallace and Hobbs (1977), p332.
Figure 2 shows solar radiation at the top of the atmosphere and the actual
radiation at sea level which has been reduced due to absorption by atmospheric
gases. The dashed curve is a blackbody at 5900K for comparison with the
solar curve outside the earth's atmosphere.
Figure 2: Solar irradiance at the top and bottom of the earth's
atmosphere, for the sun at the zenith. Shaded areas indicate absorption
by atmospheric gases. After Valley (1965).
Dr D C Griersmith
