Geoscience Reference
In-Depth Information
surface temperature often experienced on a sunny day
when a cloud temporarily cuts off the direct solar radi-
ation illustrates our reliance upon the sun's radiant
energy. How much radiation is actually reflected
by clouds depends on the amount of cloud cover and
its thickness (Figure 3.6). The proportion of incident
radiation that is reflected is termed the
albedo
, or reflec-
tion coefficient (expressed as a fraction or percentage).
Cloud type affects the albedo. Aircraft measurements
show that the albedo of a complete overcast ranges
from 44 to 50 per cent for cirrostratus to 90 per cent for
cumulonimbus. Average albedo values, as determined
by satellites, aircraft and surface measurements, are
summarized in Table 3.2 (see Note 2).
The total (or global) solar radiation received at the
surface on cloudy days is
Figure 3.5
The average annual latitudinal disposition of solar
radiation in W m
-2
. Of 100 per cent radiation entering the top of
the atmosphere, about 20 per cent is reflected back to space by
clouds, 3 per cent by air (plus dust and water vapour), and 8 per
cent by the earth's surface. Three per cent is absorbed by clouds,
18 per cent by the air, and 48 per cent by the earth.
Source
: After Sellers (1965).
S=S
0
[
b
(1 -
b
) (1 -
c
)]
where
S
0
= global solar radiation for clear skies;
c
= cloudiness (fraction of sky covered);
b
= a coefficient depending on cloud type and
thickness; and the depth of atmosphere
through which the radiation must pass.
temperature of the earth's surface would fall by some
40°C!
The atmospheric scattering, noted above, gives rise
to
diffuse
(or sky) radiation and this is sometimes
measured separately from the direct beam radiation. On
average, under cloud-free conditions the ratio of diffuse
to total (or global) solar radiation is about 0.15-0.20 at
the surface. For average cloudiness, the ratio is about
0.5 at the surface, decreasing to around 0.1 at 4 km, as a
result of the decrease in cloud droplets and aerosols with
altitude. During a total solar eclipse, experienced over
much of western Europe in August 1999, the elimination
of direct beam radiation caused diffuse radiation to drop
from 680 W m
-2
at 10.30 a.m. to only 14 W m
-2
at 11.00
a.m. at Bracknell in southern England.
Figure 3.5 illustrates the relative roles of the
atmosphere, clouds and the earth's surface in reflecting
and absorbing solar radiation at different latitudes.
(A more complete analysis of the heat budget of the
earth-atmosphere system is given in D, this chapter.)
For mean monthly values for the United States,
b
≈ 0.35,
so that
S ≈ S
0
[1 - 0.65
c
]
Table 3.2
The average (integrated) albedo of various
surfaces (0.3-0.4 µm).
Planet earth
0.31
Global surface
0.14-0.16
Global cloud
0.23
Cumulonimbus
0.9
Stratocumulus
0.6
Cirrus
0.4-0.5
Fresh snow
0.8-0.9
Melting snow
0.4-0.6
Sand
0.30-0.35
Grass, cereal crops
0.18-0.25
Deciduous
forest0.15-0.18
Coniferous
forest0.09-0.15
3 Effect of cloud cover
Tropical
rainforest0.07-0.15
Water bodies
*
0.06-0.10
Thick and continuous cloud cover forms a significant
barrier to the penetration of radiation. The drop in
Note
:
*
Increases sharply at low solar angles.
