Geoscience Reference
In-Depth Information
Table 2.7 Approximate mean albedo values for various natural surfaces
Nature of surface
Albedo
Deep water
0.04-0.08
Moist dark soils; ploughed fields
0.05-0.15
Gray soils, bare fields
0.15-0.25
Dry soils, desert
0.20-0.35
White sand; lime
0.30-0.40
Green grass and other short vegetation (e.g. alfalfa,
potatoes, beets)
0.15-0.25
Dry grass; stubble
0.15-0.20
Dry prairie and savannah
0.20-0.30
Coniferous forest
0.10-0.15
Deciduous forest
0.15-0.25
Forest with melting snow
0.20-0.30
Old and dirty snow cover
0.35-0.65
Clean, stable snow cover
0.60-0.75
Fresh dry snow
0.80-0.90
Table 2.8 Values of the emissivities
ε
s
of some natural surfaces
Nature of surface
Emissivity
Bare soil (mineral)
0.95-0.97
Bare soil (organic)
0.97-0.98
Grassy vegetation
0.97-0.98
Tree vegetation
0.96-0.97
Snow (old)
0.97
Snow (fresh)
0.99
ε
s
for different surfaces compiled from the literature (see,
for example, Van Wijk and Scholte-Ubing, 1963; Kondratyev, 1969). In many practical
applications it is simply assumed that
summarizes a few values of
1. Moreover, since
T
s
is rarely known, for
daily or longer averages over land, Equation (2.79) is often applied by using the air
temperature
T
a
instead of the surface temperature
T
s
.
The downward long-wave radiation
R
ld
can be calculated accurately on the basis
of vertical profile data of humidity and temperature. Such data are not always available
where the long-wave radiation is needed; as a result simpler methods have been developed
that rely on readily available measurements, such as air temperature and humidity near
the ground. For clear sky conditions, they are mostly based on an equation of the type
ε
s
=
T
a
R
ldc
=
ε
ac
σ
(2.80)
