Environmental Engineering Reference
In-Depth Information
ENERGY TRANSFERS AND THE GLOBAL CIRCULATION
Four main forms of energy exist in atmospheric circulation: latent heat, sensible heat,
potential energy and kinetic energy. The total energy of a unit mass of air (
E
t
) can
therefore be described as follows:
where
Lq
= latent heat content (latent heat of vaporization × specific humidity),
CpT
=
sensible heat content (specific heat of air × temperature),
gz
= potential energy
(gravitational force × height) and
V
2
/2 = kinetic energy (speed squared divided by two).
Latent heat is the quantity of heat released or absorbed, without any change of
temperature, during the transformation of a substance from one state to another (e.g. from
solid to liquid). Sensible heat can be thought of as the temperature of the atmosphere.
More specifically it is the temperature of the air (
T
) multiplied by the specific heat (
Cp
)
1
of the air at a constant pressure. Sensible heat is gained from the ground surface after the
absorption of short-wave radiation, or by the release of latent heat through condensation.
The potential energy of the atmosphere is essentially a function of its height above the
ground surface (
z
); gravity (
g
) is a constant. As air moves in the atmosphere it tends to
change its height and alter its energy content. If the air sinks slowly, the potential energy
decreases. Normally it is
Figure 3.11
Earth—atmosphere radiant energy flow system.
converted to sensible heat, and the air becomes warmer as it subsides. If the air rises, the
temperature tends to decline but the potential energy increases.
