Absorptivities for Thermal and Solar Radiation

11.5 Absorptivities for Thermal and Solar Radiation

According to law, given in Ch. 10, the absorptivity in a given waveband is equal to the emissivity in that waveband. The absorptivity needed for Eq. (1 1.14) is therefore equal to the emissivity of the surface. In Ch. 10 we give a typical value for emissivities of natural surfaces of around 0.97. Table 11.3 gives measured values for leaves, animals, and various other surfaces. Note that, except for metal surfaces, the emissivities are around the 0.97 value used in Ch. 10. We therefore continue to use this value for emissivities of natural surfaces and for sorptivities of leaves and animals. Obviously a much lower value should

be used for a metal surface. Note that a polished metal coating on a sur- face (gold, silver, or aluminum) can almost eliminate both the absorption and the emission of thermal radiation. This fact is used in the design of Thermos bottles. By silvering the glass surfaces of the bottle the

Absorptivities for Thermal and Solar Radiation

(and absorptivities) for leaves, animals, and other surfaces

T A B LE 11.3. Long-wave or thermal

maize leaf human skin tobacco leaf

snowshoe hare bean leaf

caribou

cotton leaf gray wolf sugar cane leaf

gray squirrel poplar leaf

window glass cactus

concrete

polished chrome

soil

bright aluminum

water

foil

sivity is reduced above 0.9 to below 0.05, thus almost eliminating

radiative exchange between the inner and outer bottle surfaces.

In Ch. 10 we discuss the computation of absorptivities for radiation and indicate that the absorptivity for a particular source of radiation is

the normalized integral of spectral absorptivity weighted by the spectral of the source. Figure 11.5 shows the spectral absorptivities of some leaf and animal surfaces in the shortwave region of the spectrum. While all of the surfaces show variation of absorptivity with wavelength, the leaf absorptivity changes dramatically between the visible and near infrared portions of the spectrum. In the visible, most of the radiation is absorbed, and is used to carry on photosynthesis. Absorption is somewhat lower in the green (around 0.55

part of the spectrum, resulting in

Wavelength

F I GURE 1 1.5. Spectral absorptivity of leaf, fur, feather, and skin surfaces over part of the solar spectrum (data from Gates, 1980, and Hall et

178 Radiation Fluxes in Natural Environments

T A B LE 11.4. Shortwave absorptivities of leaves and animals (from Gates, 1980).

Leaves Mammals

silver maple bison american beach

wolf sunflower

cat (white) cottonwood

bobcat cottonwood (yellow)

Reptiles Birds

alligator Stellar's jay

lizard sparrow (dorsal)

Humans quail (dorsal)

Eurasian quail egg

Negroid white swan

the characteristic green color of vegetation, but overall, approximately

85 percent of the incident visible radiation is absorbed while about 15 percent of the NIR is absorbed. The NIR wavelengths are not useful for biochemical processes and are largely reflected or transmitted by the leaf.

Integration of data like that shown in Fig. 1.5, with weighting accord- ing to the solar spectrum (Fig.

results in the shortwave absorptivitiy for solar radiation. Table 11.2 shows solar reflectivities for various types of ground cover. Since all of the incident radiation is either reflected or ab-

sorbed, the absorptivity of these surfaces can be computed as = 1 -

where comes from Table 1 1.2. Representative animal and leaf tivities are given in Table 11.4. Gates (1980) gives a more comprehensive table.

It appears that shortwave absorptivities of leaves are around 0.5, so about half of the incident solar radiation is absorbed. Animals have a wide range of absorptivities ranging from 0.18 for eggs to around 0.9 for black or dark brown coats. Even white coats like the white swan and white cat absorb around 40 percent of the incident radiation. Comparing the solar absorptivities of leaves

0.5) from Table 11.4 with canopies 0.8) from Table 11.2 reveals a surprising difference. The higher absorptivity of canopies arises because of multiple reflections among leaves in a canopy and depends on the architecture of the canopy.