Geoscience Reference
In-Depth Information
Table 5.1
Typical all day average values of the albedo for selected
land covers.
Surface Type
Typical Value of albedo
Open water surfaces
0.08 (
8%)
∼
∼
Fresh snow
∼
0.8 (
∼
80%)
Dirty, old snow
0.4 (
40%)
∼
∼
Bare soil and agricultural crops
∼
0.23 (
∼
23%)
Forest
0.12 (12%)
∼
Integrated radiation parameters for natural surfaces
As mentioned above, most of the solar (shortwave) radiation in the Earth system
is in the wavelength range 0.15-4.0
m while most of the terrestrial (longwave)
radiation is in the wavelength range 3-100
μ
m. Because there is so little overlap
between the wavelengths that define solar and terrestrial radiation, it is possible to
define integrated values of reflection coefficient and emissivity for natural (gray)
terrestrial surface, as follows:
μ
The albedo, a,
of a natural surface is the integrated reflectivity of the surface
for radiation incident over the frequency range 0.15-4.0
●
μ
m
The surface emissivity,
, of a natural surface is the integrated emissivity of
the surface over the frequency range 3-100
e
●
μ
m
The surface emissivity of many natural surfaces is in the range 0.90-0.99, or
90-99%. The value of daily average albedo for natural surfaces varies and depends
on the nature of the surface. Table 5.1 gives typical values for some terrestrial
surfaces important in hydrometeorology. Note that the albedo for forests is about
half that typical of bare soil and agricultural crops. In fact, this is more generally
the case for 'tufty' vegetation that has a rough canopy with significant depressions
that can trap solar radiation more easily.
More solar radiation is reflected when the angle of incidence of the solar beam
is low in the morning and evening, and the albedo is therefore substantially greater
at these times. Figure 5.5 shows some examples of how albedo varies with solar
altitude. However, because there is usually much more incoming solar radiation in
the middle of the day than in the early morning and late evening, the daily average
value of albedo is biased toward lower midday values.
The values for albedo given in Table 5.1 are typical daily average values but it is
important to recognize that the reflection coefficient for solar radiation can change
significantly from place to place even if the vegetation cover is the same. Table 5.2
shows the observed range of values for albedo and emissivity for different surfaces.
