reflection can be neglected, as opposed to when the cover is sparse, or the
ground covered with snow. The albedo of natural surfaces varies during the
day, peaks in the morning, and mid-afternoon and reaches a minimum
around noon (Oke 1992). The variation of albedo with the zenith angle is
exponential.
This variation is typical of specular reflection, as can be seen from curves of
reflection versus solar height (the complementary angle of solar inclination) in clear
or cloudy skies over water surfaces (Oke 1992; Monteith and Unsworth 1991);
(iv) The architecture of the canopy, which is the most important factor influencing
the dynamics of canopy radiation. The albedo of vegetation is lower than the
reflectivity for the individual leaves since the reflection depends not only on
the radiative properties of the individual components, but also on the structure
and solar elevation angle. The latter two determine the penetration and
retention of incident radiation, as well as the shade provided by the canopy
components (Oke 1992).
Significant variations of radiation that occur in a clear sky day because of alternating light beams and shaded areas are determined by the canopy structure, as well
as plant and leaf distribution. The fraction of radiation intercepted by plant canopy
depends on the leaf area index, defined as the projected area of leaves per unit area
of soil and the spatial distribution of foliage relative to the radiation direction. The
scattered radiation fraction depends on the optical properties of anatomical components of the leaves, such as cuticles, cell walls, and pigments (Monteith and
Unsworth 1991).
The highest albedos are recorded on flat surfaces such as lawns. For vegetation
layers, with heights ranging between 50 and 100 cm that fully cover the soil, the
albedo ranges between 0.18 and 0.25. For forest canopies, the albedo is about 0.1.
When the ground is partially covered by the vegetation or when leaves are partially
dry, the albedo will depend on foliage reflection and the reflection from the ground.
Albedo differences between the different forms of vegetation are due to the complexity of reflection and the scattering of radiation within.
The reflection coefficient for various forms of vegetation also depends on the
zenith angle as a key component of specular reflection (Lee 1978). The minimum
albedo values of the areas under crops are at noon when the sun is at the zenith and
the maximum values occur when the sun approaches the horizon, due to specular
reflection of vegetation. Dependence of albedo on solar height also helps to explain
the lower albedo values in tropical areas, in relation to the higher latitude areas. On
days with cloudiness, or under any other conditions, wherein the diffuse component
of the solar radiation is significant, the daily variation of albedo is lower than in
clear sky conditions (Lee 1978).
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6 Heat and Mass Transfer Processes
ground covered with snow. The albedo of natural surfaces varies during the
day, peaks in the morning, and mid-afternoon and reaches a minimum
around noon (Oke 1992). The variation of albedo with the zenith angle is
exponential.
This variation is typical of specular reflection, as can be seen from curves of
reflection versus solar height (the complementary angle of solar inclination) in clear
or cloudy skies over water surfaces (Oke 1992; Monteith and Unsworth 1991);
(iv) The architecture of the canopy, which is the most important factor influencing
the dynamics of canopy radiation. The albedo of vegetation is lower than the
reflectivity for the individual leaves since the reflection depends not only on
the radiative properties of the individual components, but also on the structure
and solar elevation angle. The latter two determine the penetration and
retention of incident radiation, as well as the shade provided by the canopy
components (Oke 1992).
Significant variations of radiation that occur in a clear sky day because of alternating light beams and shaded areas are determined by the canopy structure, as well
as plant and leaf distribution. The fraction of radiation intercepted by plant canopy
depends on the leaf area index, defined as the projected area of leaves per unit area
of soil and the spatial distribution of foliage relative to the radiation direction. The
scattered radiation fraction depends on the optical properties of anatomical components of the leaves, such as cuticles, cell walls, and pigments (Monteith and
Unsworth 1991).
The highest albedos are recorded on flat surfaces such as lawns. For vegetation
layers, with heights ranging between 50 and 100 cm that fully cover the soil, the
albedo ranges between 0.18 and 0.25. For forest canopies, the albedo is about 0.1.
When the ground is partially covered by the vegetation or when leaves are partially
dry, the albedo will depend on foliage reflection and the reflection from the ground.
Albedo differences between the different forms of vegetation are due to the complexity of reflection and the scattering of radiation within.
The reflection coefficient for various forms of vegetation also depends on the
zenith angle as a key component of specular reflection (Lee 1978). The minimum
albedo values of the areas under crops are at noon when the sun is at the zenith and
the maximum values occur when the sun approaches the horizon, due to specular
reflection of vegetation. Dependence of albedo on solar height also helps to explain
the lower albedo values in tropical areas, in relation to the higher latitude areas. On
days with cloudiness, or under any other conditions, wherein the diffuse component
of the solar radiation is significant, the daily variation of albedo is lower than in
clear sky conditions (Lee 1978).
200
6 Heat and Mass Transfer Processes