Transfer processes in animal coats. I. Radiative transfer

Abstract

A silicon cell and a radiometer were used to measure the transmission of short wave (0.4-1 $\mu $m) and long wave (3-100 $\mu $m) radiation through the coats of sheep and cattle. The reflectivity of the same coats was measured in a spectrophotometer. Measurements of transmission and reflexion were interpreted in terms of a theory of radiation scattering developed (for vegetation) by Cowan (1971), assuming that radiation striking a single hair could be absorbed or scattered either towards or away from the skin. One of the parameters used in the theory is the fraction of radiation intercepted by a ray in unit depth of coat (p). For diffuse radiation the appropriate mean value of the interception function ($\overline{p}$) is approximately twice the value of p for a ray at normal incidence. The value of $\overline{p}$ ranged from about 9 cm$^{-1}$ for sheep's fleece to 36 cm$^{-1}$ for calf and deer coats. In the short wave spectrum, mean reflexion coefficients for the whole coat ranged from 0.30 for Welsh Mountain Sheep (black fleece) to 0.79 for Dorset Down Sheep (white fleece); corresponding values of the absorption coefficient for individual hairs were 0.02 and 0.002. On the basis of these and related figures, the absorption of solar radiation by the skin surface was evaluated for different combinations of fleece and skin colour. The combination of a light fleece and a dark skin is a trap for solar radiation because the scattering by hair is predominantly forwards.