Air movement and heat loss from sheep. II. Thermal insulation of fleece in wind.

Penetration of an animal's coat by wind reduces its thermal insulation and increases heat loss to the environment. From studies of the sensible heat loss from a life-sized model sheep covered with fleece, the average fleece resistance -rf(s cm-1) was related to windspeed u (m s-1) by 1/-rf(u) = 1/-rf(0)+cu, where c is a dimensionless constant. As c is expected to be inversely proportional to coat depth l, the more general relation -k(u) = -k(0)+c'u was evaluated, where -k = l/-rf is the thermal diffusivity (cm2 s-1) of the fleece and c' = cl is another constant (cm). The orientation of the model to the wind had little effect on the bulk resistance of the fleece, but the resistance on the windward side was substantially lower than on the leeward side.