Heat and energy in the atmosphere - Fundamentals of the Physical Environment - page 47

Plate 3.1 Polythene sheeting for horticulture in southern Spain changing surface albedo.
Photo: Peter Smithson

Earth, a mean figure of 19 per cent of the original insola-
tion is reflected by clouds. The lowest and thickest clouds
tend to reflect most, while the thin, high-level ice clouds
have an albedo of only about 30 per cent.
By now, the beam has reached the ground surface with,
as a global average, about 50 per cent of its original energy.
Even then, not all of it is absorbed, as the surface itself
has an albedo. The global average albedo represents some
6 per cent of the radiation at the top of the atmosphere,
so the loss is not great. However, the figure may seem large
when expressed as a percentage of the radiation actually
reaching the surface. For example, the albedo of freshly
fallen snow may reach as high as 90 per cent ( Table 3.2 ).
The greatest variability is over water. When the sun is high
in the sky, water has a very low albedo. That is why oceans
appear dark on satellite photographs ( Plate 3.2 ). At low
angles of the sun, as at dawn or in midwinter in temperate
and sub-polar latitudes, the albedo may reach nearly
80 per cent.
The sunlight reaching Earth's surface which is not
reflected by Earth is absorbed and converted into heat
energy. The distribution of energy received at the surface

is shown in Figure 3.3 . In summary, incoming radiation
can be absorbed (in the atmosphere and at the surface),
scattered (in the atmosphere) or reflected (by clouds and

Table 3.2 Albedos for the short-wave part of the
spectrum

Surface Albedo (%)

Water (zenith angles above 40°) 2-4

Water (angles less than 40°) 6-80

Fresh snow 75-90

Old snow 40-70

Dry sand 35-45

Dark, wet soil 5-15

Dry concrete 17-27

Black road surface 5-10

Grass 20-30

Deciduous forest 10-20

Coniferous forest 5-15

Crops 15-25

Tundra 15-20