Geoscience Reference
In-Depth Information
Table B.2
Overview of different variables used to indicate the amount of water
vapour in air
Symbol
Name
Unit
Description/remark
ρ
v
Absolute
humidity
kg m
-3
Water vapour density; often used in sensors based on
absorption of radiation
e
Water vapour
pressure
Pa
Partial pressure of water vapour (often used in
models for evaporation)
r
Mixing ratio
kg kg
-1
Mass of water vapour as a fraction of mass of dry air:
r
=
ρ
ρ
v
d
(conserved for adiabatic processes)
q
Speciic
humidity
kg kg
-1
Mass of water vapour as a fraction of mass of
moist air
q
=
ρ
ρ
ρ
ρρ
v
=
v
(conserved for adiabatic
+
d
v
processes)
T
v
Virtual
temperature
K
Temperature that
dry
air should have to have the
same density as air with a given moisture content:
TT
1061 . Used when the buoyancy effect of
water vapour is relevant.
≈+
(
.
q
)
v
RH
Relative
humidity
—
Vapour pressure as a fraction of the vapour pressure
at saturation for a given temperature:
RH
e
eT
=
sat
()
(see Eq. (
B.20
) for a deinition of
e
sat
). Easily
measured with hair hygrometers. Relevant since
many natural materials are sensitive to relative
humidity.
T
d
Dew point
temperature
K
Temperature to which air needs to be cooled (at
constant pressure) to reach saturation, i.e. where
eeT
=
sat
(
.Relevant to predict effect of cooling at
constant pressure. Can be measured with a dew point
mirror.
T
w
Wet bulb
temperature
K
The temperature to which the wet bulb of a
psychrometer will cool when exposed to air with
a given moisture content (heat will be extracted to
provide energy for evaporation). Can be measured
with a psychrometer.
Through the deinitions, mixing ratio and speciic humidity are directly related:
r
q
=
+1
(B.17)
r
Because both
r
and
q
are much less than 1, for many practical applications
q
and
r
can be assumed to be identical. The fact that both
q
and
r
are conserved variables for
adiabatic processes is due to the fact that both the total density, and the density of
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