Geoscience Reference
In-Depth Information
models are being described by Mendes (
1999
). An approximate relation between
water vapor pressure and delay reads
0
.
217
p
w
T
L
z
w
≈
Δ
.
(49)
Assuming an isothermal atmosphere with exponential decrease of water vapor pres-
sure
p
w
, and assuming that water vapor exists until a height of 2 km, we get an
approximation for the wet delay as a function of water vapor pressure at the Earth's
surface
p
w
0
748
p
w
0
L
z
w
≈
Δ
T
0
.
(50)
An even simpler way is a rule of thumb that suggests that the wet zenith delay in cm
equals the water vapor pressure in hPa at the Earth's surface
L
z
w
[cm]
Δ
≈
.
p
w
0
[hPa]
(51)
In any case, information of water vapor pressure and/or temperature at the surface
has to be known. If no surface meteorological observation is available, we can use the
simple model of the standard atmosphere where
p
w
can be calculated as a function
of the relative humidity
f
, i.e.
f
100
exp
000256908
T
2
p
w
=
(
−
37
.
2465
+
0
.
213166
T
−
0
.
).
(52)
3.2.2 Conversion of Zenith Wet Delays to Precipitable Water
The zenith wet delay can be related to the amount of integrated water vapor above the
station. Following Eq. (
46
) and using the expression for integrated mean temperature
T
m
(Bevis et al.
1992
)
h
0
(
s
(
e
e
T
Z
−
1
T
Z
−
1
)
d
z
)
d
s
w
w
T
m
=
s
(
d
s
≈
h
0
(
d
z
,
(53)
T
2
Z
−
1
e
T
2
Z
−
1
e
)
)
w
w
we can write
10
−
6
k
2
+
e
T
Z
−
1
d
s
k
3
T
m
Δ
L
w
=
.
(54)
w
s
Applying the ideal gas laws, Eq. (
54
) can be reformulated as
10
−
6
k
2
+
R
M
w
k
3
T
m
Δ
L
w
=
s
ρ
w
d
s
.
(55)
