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(a)
500
400
300
200
100
22 May, cloudy
23 May, sunny
0
0
3
6
9
12
15
18
21
24
Time (hour UTC)
(b)
500
400
300
200
100
22 May, cloudy
23 May, sunny
0
0
3
6
9
12
15
18
21
24
Time (hour UTC)
Figure 2.14
Observations of longwave radiation at Haarweg Meteorological station
for a clear day May 23, 2007) and a cloudy day (May 22, 2007). (
a
) Downwelling
longwave radiation. (
b
) upwelling longwave radiation.
hundred meters (of which the temperature correlates with the temperature at stan-
dard level). Furthermore, the atmosphere is assumed to be a grey body, with apparent
emissivity
ε
a
:
↓
=
εσ
a
4
(2.24)
L
T
a
where σ is the Stefan-Boltzmann constant (= 5.67 10
-8
W m
-2
K
-4
). For cloudless con-
ditions, water vapour is the most variable, radiatively active, constituent. This obser-
vation is the basis for many empirical models for
ε
a
(see, e.g., Crawford and Duchon,
1999
). One often used model for
ε
a
for clear conditions (
ε
a,clear
) is the approximation
of Brunt (
1932
):
εε
a
=
=
cce
1
(2.25)
a,clear
2
a
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