Geoscience Reference
In-Depth Information
0.5
8
0.4
6
0.3
4
0.2
2
Figure 2.2 Variation in
saturated vapor pressure of water
and the gradient of that variation
as a function of temperature
in °C.
0.1
0
0.0
0
10
20
T c C
30
40
0
10
20
T c C
30
40
Figure 2.2 illustrates how e sat and
change as a functions of T C . Both variables
change substantially over the normal range of temperatures. The fact that e sat and
Δ
Δ
have specified relationships with temperature is very important in hydrometeor-
ology: it means that additional equations are available to link the surface exchanges
of water vapor and heat.
At a particular temperature, the density of water vapor in saturated air,
r v,sat , can
be specified from the saturated vapor pressure using Equation (2.6). Consequently,
corresponding values of saturated mixing ratio, r sat , and saturated specific humid-
ity, q sat , can be defined by substituting this value of
r v,sat into Equations (2.2) and
(2.3), respectively.
Measures of saturation
Figure 2.3 is helpful, not only because it defines the extent to which the atmosphere
is saturated, but also because later it is used in important methods to measure
the vapor pressure of moist air. This diagram is for the example case of an atmos-
phere with a temperature of 35
C that is 60% saturated.
Probably the most common way to specify the extent to which air is saturated is
to specify its relative humidity, RH . Relative humidity is defined as the ratio of the
actual vapor pressure of the air to the saturation vapor pressure at air temperature
and is normally expressed as a percentage. To good accuracy the relative humidity
can also be calculated as the ratio of the mixing ratio of the air to saturation mixing
ratio at air temperature, or as the ratio of specific humidity of the air to the satura-
tion specific humidity at air temperature, thus:
°
e
r
q
RH
=
100
%
=
100
%
=
100
%
e
r
q
(2.19)
sat
sat
sat
A second important measure of atmospheric humidity content is the vapor pressure
deficit, D . The vapor pressure deficit is defined as the difference between saturation
vapor pressure at air temperature and the vapor pressure of the air, i.e.:
Search WWH ::




Custom Search