Heat balance and thermal resistances of sheep's fleece

Abstract

Measurements of the net radiation and radiative surface temperature of sheep's fleece 0.5-10 cm deep were used to analyse the local sensible heat balance. Substantial variations in the heat flux at the skin surface were produced by changing the net radiation at the fleece surface in the range -50 to +400 W m-2 representing outdoor conditions. Coat resistances of clean fleeces were linearly related to coat depth and the thermal conductivity was about 0.19 W m-1 degrees C-1. Sensible heat exchange at the skin depends on the net radiation outside the coat, a quantity which has not usually been measured in studies of animal heat balance. A simple resistance analogue is presented, describing the local energy balance as the current through the tissue, coat and aerodynamic resistances in series, modified by the net radiation, considered as a current injected at the coat surface. This analogue is employed to predict the influence of radiative load on the animal's local heat balance, and in particular on the extent of the thermoneutral zone.