28 3 Radiation Balance near the Surface
significant changes in the surface energy balance and hence in the micro-
or macroclimate.
For
example, deliberate modification of albedo by large-
scale surface covering has
been
proposed as a means of increasing precip-
itation in certain arid regions.
It
has also been suggested that large, fre-
quent oil spills in
the
ice-
and
snow-covered arctic region could lead to
significant changes in the regional energy balance and climate.
Snow is
the
most
effective reflector (high albedo) of shortwave radia-
tion. On the
other
hand, water is probably the poorest reflector (low
albedo), while ice falls between snow and water. Since large areas of earth
are
covered
by water,
sea
ice, and snow, significant changes in the snow
and ice
cover
are likely to cause perceptible changes in the regional and,
possibly, global albedo.
Most
bare
rock, sand, and soil surfaces reflect 10-45% of the incident
shortwave radiation, the highest value being for the desert sands. Albedos
of
most vegetative surfaces fall in the range of 10-25%. Wetting of the
surface by irrigation or precipitation considerably lowers the albedo,
while snowfall increases it. Albedos of vegetative surfaces, being sensi-
tive to the solar elevation angle, also show some diurnal variations, with
their minimum values around noon and maximum values near sunrise and
sunset (see, e.g., Rosenberg
et al., 1983, Chapter 1).
Shortwave reflectivities of clouds vary
over
a wide range, depending on
the cloud type, height,
and
size, as well as on the angle of incidence of
radiation (see, e.g., Welch
et al., 1980).Thick stratus, stratocumulus, and
nimbostratus clouds are good reflectors
(a = 0.6-0.8, at normal inci-
dence), large cumulus clouds are moderate reflectors
(a = 0.2-0.5), and
small cumuli are
poor
reflectors (a < 0.2). Only a small part of solar
radiation may reach the ground in cloudy conditions. A significant part of
the reflected radiation from the surface may be reflected back by clouds.
In fact, shortwave radiation is likely to undergo multiple reflections be-
tween the surfaces
and
bases of clouds, thereby increasing the effective
albedo of the surface.
3.2.3
DIFFUSE
RADIATION
Diffuse or sky radiation is
that
portion of the solar radiation that
reaches the
earth's
surface after having been scattered by molecules and
suspended particulates in the atmosphere. In cloudy conditions it also
includes the portion of the shortwave radiation which is reflected by the
clouds.
It
is the incoming shortwave radiation in shade. Before sunrise
and after sunset, all shortwave radiation is in diffuse form. The ratio of
diffuse to the total incoming shortwave radiation varies diurnally, season-
ally, and with latitude. In high latitudes, diffuse radiation is very impor-