Environmental Engineering Reference
In-Depth Information
Figure 3.3
Modification of short- and long-wave radiation by
the atmosphere and the surface. Figures are expressed as a
percentage of incoming short-wave radiation at the top of the
atmosphere based on a global mean.
wards. As there is less water vapour at higher levels, absorption by the atmosphere is less
and proportionally more is lost to space.
GLOBAL RADIATION BALANCE
Taking Earth as a whole, we know that no part is getting warmer (apart from the possible
'enhanced' greenhouse effect) or cooler and so there must be an overall balance. More
short-wave radiation appears to be absorbed by Earth than leaves it by a mixture of short
and long-wave radiation. The surface seems to be gaining heat (Figure 3.3). Similarly, the
atmosphere seems to be losing heat. If radiation were the only process operating, Earth's
surface should be getting warmer and the atmosphere cooler. They do not do so because,
in addition to radiation, there are thermal energy transfers in the form of convective heat
exchanges. Many of them take place through evaporation and are discussed later in this
chapter.
