Thermal insulation of short lengths of Merino fleece

Abstract

The thermal insulation of the air and of short lengths of fleece, such as might be left after shearing, has been determined with a hot plate apparatus in various sites on the trunk of a model sheep. The model was placed in a wind tunnel and could be set at any angle to the wind. The insulation of the fleece in still air was about 1.5 clo units per cm, being similar to that found by others for the pelage of many mammals and for the best clothing materials. The superiority in still air of the insulation of fleece plus air for a 1.27 cm (½ in.) length over that of 0.48 cm (3/16 in.) length was maintained in wind. The effect of wind on insulation of the fleece alone at an individual site depended on the angle of the sheep to the wind. The angle of incidence of the wind was as important as the rate of air movement at the site. For the trunk of the sheep as a whole, angle to wind was not important. The longer lengths of fleece were more affected than the shorter lengths. Thus, at a wind speed of 10-11 m.p.h. the insulation of a 0.48 cm length averaged 1.34 clo/cm, but it was only 1.10 clo (high midside site only) for a 2.54 cm (1 in.) length. The insulation of the air was not greatly different from that found by previous workers for man. The reasons for the difference are discussed, and the application of the present data to fleeced animals and field conditions is explained. For this, the convection term has been calculated so that the data can be used in conjunction with other values of radiative exchange appropriate for field conditions. The relation between fleece length and net cooling by evaporation from the tips of the fleece is discussed in the light of the experimental results, and also the relation of these results to Priestley's (1957) calculation of solar heat load on a sheep.