Geoscience Reference
In-Depth Information
ratio of the density of water vapor to that of moist air, called the
specific
humidity
(
q
). The vapor pressure of air (
e
) is also a measure of atmospheric
water content.
Ideal gas law
: the temperature (
T
), volume (
V
) and pressure (
P
) of a gas
are related by the
ideal gas law
which can be written as
PV
●
=
r
g
R
g
T
where
r
g
and
R
g
R
/
M
g
are the density and (gas-specific) gas constant, respectively,
and
M
g
is its gram molecular weight. When applied to both the dry air and
water vapor portions of moist air, to good accuracy this gives the result
q
=
=
r
=
(0.622
e
/
P
).
Virtual temperature
:
T
v
T
(1+0.61
q
) is the temperature that dry air would
have if it had the same density and temperature as the moist air.
=
●
Saturated vapor pressure
: the maximum vapor pressure of air when
saturated,
e
sat
, and the rate of change of
e
sat
with temperature,
●
Δ
, are both
well-defined functions of the temperature.
Measures of saturation
: two measures in common use are the relative
humidity,
RH
●
=
(100
e
)/
e
sat
, and the vapor pressure deficit
D
=
(
e
sat
-
e
).
Measuring the vapor pressure
: two ways to measure vapor pressure are:
—
Dew point hygrometer
: Dew point is obtained as the temperature of an
initially clear mirror cooled until its surface becomes clouded by dew.
Equation (2.17) is then applied to calculate
e
from
T
dew
.
—
Wet bulb psychrometer
: Two (preferably aspirated) thermometers, one
dry and one wet, measure the dry bulb and wet bulb temperatures,
T
dry
and
T
wet
, respectively; vapor pressure is calculated from
e
●
=
e
sat
(
T
wet
) -
γ
(
T
dry
-
T
wet
) where
e
sat
(
T
w
) is the saturated vapor pressure
at wet bulb tem-
perature
and
γ
is the
psychrometric constant
.
