Environmental Engineering Reference
In-Depth Information
Figure 2.3
Energy transfer during the change in state of water
released (see Figure 2.3). The water vapour may
travel over great distances in the atmosphere in
the period between the absorption and re-release
of the energy, and in this way energy absorbed
in one location is transported elsewhere in the
system.
Water is also involved in the earth's energy
budget through its ability to absorb and reflect
radiation. As vapour, it contributes to the
greenhouse effect by absorbing terrestrial
radiation, whereas in its liquid and solid forms it
can be highly reflective. As clouds in the air or
snow on the ground, it may reflect as much as
90 per cent of the solar radiation it intercepts.
That radiation, reflected back into space, makes
no contribution to the energy requirements of
the system. In contrast, clouds can also help to
retain terrestrial radiation by reflecting it back
to the surface.
Water is an integral part of many global
environmental issues such as drought,
desertification and acid rain. In addition, because
of its role in the earth's energy budget, any change
in the distribution of water in the earth/
atmosphere system might well augment or
uneven, as a result of regional variability in the
dynamic processes which produce precipitation.
Intense thunderstorms can yield 25 mm of
precipitation in a matter of minutes, whereas the
same amount may take several months, or even
years, to accumulate under more stable
atmospheric conditions. Variations such as these
account for annual precipitation totals which
range from virtually nothing, in some of the
world's deserts, to as much as 4,000 mm, in the
monsoon lands of the tropics. Precipitation in
excess of the quantities normally in the air is made
possible by the hydrologic cycle, which circulates
water through the earth/atmosphere system, and
regularly replenishes the atmospheric reservoir
(see Figure 2.2).
Water is unique among the constituents of the
atmosphere in that it is capable of existing as
solid, liquid or gas, and of changing readily from
one state to another. It becomes involved in
energy transfer as a result of these changes. For
example, energy absorbed during the conversion
of liquid water to water vapour is retained by
the latter, in the form of latent heat, until the
process is reversed. The stored energy is then
