Geoscience Reference
In-Depth Information
• The speciic humidity distribution has features in common with the surface
temperature distribution
(Fig. 2.6).
For example, higher specific humidity
generally accompanies the higher temperatures of the western Pacific and
Atlantic compared with the eastern parts of the ocean basins.
• As is the case for surface temperature, seasonality in speciic humidity is
greater over land surfaces.
Another measure of water vapor content is the
vapor pressure
,
e
, defined
as the partial pressure exerted by water vapor molecules in a volume of moist
air. Consider a layer of moist air overlying a plane surface of liquid water and
exchanging water molecules with the surface through evaporation and con-
densation. For a given temperature of this atmosphere/water system, the value
of the equilibrium vapor pressure, that is, the vapor pressure when the rate
of evaporation from the liquid water surface is equal to the rate of condensa-
tion onto the surface, is called the
saturation vapor pressure
,
e
s
. Atmospheric
vapor pressure is less than the saturation vapor pressure when evaporation
is restricted, when the atmospheric circulation diverges moisture, or when a
state of equilibrium has not been achieved. Air can also become supersaturated
when
condensation
(the phase change of water from vapor to liquid phase) is
somehow inhibited, for example, by the absence of condensing surfaces.
Because evaporation and condensation rates are temperature dependent, the
saturation vapor pressure also depends on temperature, and that relationship is
expressed by the Clausius-Clapeyron equation. For temperatures (in °C) typi-
cal of earth systems,
17.67
T
eT
() 0.6112
,
exp
,
(2.8)
c
m
S
T
+
243.5
as plotted in
Figure 2.32.
The relationship is not linear. Rather, the saturation
vapor pressure becomes more sensitive to temperature under warmer condi-
tions, for example, in the tropics.
Relative humidity
,
RH
, is the ratio of the
vapor pressure to the saturation vapor pressure.
80
70
60
Saturation
vapor
pressure
50
40
30
20
10
Figure 2.32 Saturation vapor
pressure as a function of
temperature.
-30
-40
-20
-22
-10
-4
0
32
10
50
20
68
30
86
40
104
(°C)
(°F)
Temperature
