Geoscience Reference
In-Depth Information
Figure 3.12
Average annual snow-cover duration (months).
Source
: Henderson-Sellers and Wilson (1983).
A large proportion of the incoming solar radiation is
reflected back into the atmosphere without heating the
earth's surface. The proportion depends upon the type of
surface (see Table 3.2). A sea surface reflects very little
unless the angle of incidence of the sun's rays is large. The
albedo for a calm water surface is only 2 to 3 per cent for
a solar elevation angle exceeding 60°, but is more than
50 per cent when the angle is 15°. For land surfaces,
the albedo is generally between 8 and 40 per cent of the
incoming radiation. The figure for forests is about 9 to
18 per cent according to the type of tree and density
of foliage (see Chapter 12C), for grass approximately
25 per cent, for cities 14 to 18 per cent, and for desert
sand 30 per cent. Fresh snow may reflect as much as
90 per cent of solar radiation, but snow cover on vege-
tated, especially forested, surfaces is much less reflective
(30 to 50 per cent). The long duration of snow cover on
the northern continents (see Figure 3.12 and Plate A)
causes much of the incoming radiation in winter to
spring to be reflected. However, the global distribution of
annual average surface albedo (Figure 3.13A) shows
mainly the influence of the snow-covered Arctic sea ice
and Antarctic ice sheet (compare Figure 3.13B for
planetary albedo).
The global solar radiation absorbed at the surface is
determined from measurements of radiation incident on
the surface and its albedo (
a
). It may be expressed as
S(100 -
a
)
where the albedo is a percentage. A snow cover will
absorb only about 15 per cent of the incident radiation,
whereas for the sea the figure generally exceeds 90 per
cent. The ability of the sea to absorb the heat received
also depends upon its transparency. As much as 20
per cent of the radiation penetrates as far down as
9 m (30 ft). Figure 3.14 illustrates how much energy
is absorbed by the sea at different depths. However,
the heat absorbed by the sea is carried down to
considerable depths by the turbulent mixing of water
masses by the action of waves and currents. Figure 3.15,
for example, illustrates the mean monthly variations
with depth in the upper 100 metres of the waters of
the eastern North Pacific (around 50°N, 145°W); it
shows the development of the seasonal thermocline
under the influences of surface heating, vertical mixing
and surface conduction.
A measure of the difference between the subsurfaces
of land and sea is given in Figure 3.16, which shows
ground temperatures at Kaliningrad (Königsberg) and
sea temperature deviations from the annual mean at
various depths in the Bay of Biscay. Heat transmission
in the soil is carried out almost wholly by conduction,
and the degree of conductivity varies with the moisture
content and porosity of each particular soil.
