Environmental Engineering Reference
In-Depth Information
RELEASE OF ENERGY
Where the state of water changes to a lower energy level (i.e from vapour to liquid or
from liquid to solid) it will release the same quantity of energy that was originally used
when it was raised to the higher energy state. This is very important in our atmospheric
heat balance. Water that is evaporating from the surface will extract energy from that
surface, where there is usually a surplus anyway. Eventually the vapour will condense in
the atmosphere, probably as a cloud droplet, releasing latent heat originally extracted
from the surface and so helping to warm the atmosphere. This can take place well away
from the original evaporation point, so evaporation can transfer heat energy both
vertically and horizontally.
Much of Earth's surface is covered by oceans, where evaporation takes place
continuously. Even a large proportion of the land surface is moist much of the time.
Consequently the role of latent heat in balancing the heat budget of Earth is vital. Latent
heat transfer by convection carries about one-fifth of the energy of incoming solar
radiation back to the atmosphere (Figure 3.10).
The heat used for evaporation over land areas depends upon the availability of
moisture and energy. In polar regions it is small, but it increases equatorwards, reaching a
maximum in the moist equatorial forests of South America, central Africa and Indonesia.
Over the desert areas there is little moisture available and evaporation is insignificant.
Figure 3.10
Global distribution of the vertical transfer of
latent heat. Units are W m
-2
.
Source: After Budyko et al. (1962).
