Geoscience Reference
In-Depth Information
0
.
622
·
e
s
=
e
.
(4)
p
−
0
.
378
·
•
The mixing ratio
m
is the ratio of the density of water vapor and dry air:
.
·
0
622
e
m
=
e
.
(5)
−
p
•
The relative humidity
f
in % is often used to characterize humidity:
e
E
·
f
=
100
.
(6)
The saturated water vapor pressure can be determined with the formula byMagnus
(Eq.
2
).
•
The dew point
is the temperature at which enough water vapor is in the air to
have saturation (equilibrium). The dew point is found by inverting the formula by
Magnus (Eq.
2
).
τ
•
Virtual temperature: The explanation of the virtual temperature follows later (see
1
378
e
p
T
v
=
T
·
+
0
.
.
(7)
2.3 Gas Laws
The most important properties of gases are understandable if we are aware that
these molecules are in constant motion. The statistical averages of these movements
determine the macroscopic state quantities, which are volume
V
, pressure
p
, and
temperature
T
. The consequences of the molecular movements are temperature and
diffusion, the latter referring to the adjustment of density differences when the gases
try to uniformly fill the whole space. For the investigations here, we consider ideal
gases: Molecules are considered as mass points (without volume), and the van der
Waals forces are neglected. These assumptions are generally reasonable.
The pressure
p
of a gas is based on the impact of the gas molecules. If a gas
is the mixture of several gases, then each exerts that pressure as if it were alone.
The total pressure is the sum of the partial pressures of the individual constituents
(Dalton's law):
p
=
p
i
.
(8)
i
As mentioned above, the state of a gas is determined by
p
,
T
, and
V
. With a given
amount of gas, two quantities can be varied whereas the third quantity is determined
by the other two. In thermodynamics the gas law
