Geoscience Reference
In-Depth Information
where
T
v
is called the
virtual temperature
. It is the temperature that dry air
would have if it had the same density and pressure as the moist air, and is given by:
⎛ ⎞
=+ −
⎡
⎤
R
TT
R
r
r
v
v
1
1
(2.12)
⎢
⎥
⎜
⎟
v
⎝
⎠
⎢
⎥
⎣
⎦
d
a
Substituting Equation (2.2) and recalling that (
R
d
/
R
v
)
=
0.62, this last equation
becomes:
v
TT r
=+
⎡
10.61
⎤
(2.13)
⎣
⎦
and because
q
and
r
are numerically similar, it is usually acceptable also to write:
v
TT q
=+
10.61
⎡
⎤
(2.14)
⎣
⎦
In the next chapter we use this definition of virtual temperature to adjust the
temperature profile of the atmosphere so as to compensate for the effect on atmos-
pheric buoyancy of density variations associated with height-dependent changes
in water vapor concentration.
Saturated vapor pressure
The net evaporation rate from a water surface is the difference between two
exchange rates: the rate at which molecules are being 'boiled off ' from the water
surface minus the rate at which molecules of water already present in the air above
the surface are recaptured back into the water, see Fig. 2.1.
Water vapor
Condensation
Vaporization
Figure 2.1
Vaporization and
capture components of the
evaporation process.
Liquid water
