Wind chill in sheep: its estimation from meteorological records

Wind chill can be calculated in two ways from the estimate of sensible (non-evaporative) heat loss (H_n) that a sheep experiences as a result of its exposure to the weather variables of air temperature (T_a), wind speed (V_a), sunshine, cloud and rain. By one method, that part of the heat loss that is due to wind (H_v) is calculated; H_v varies with the fleece depth, which provides the animal with the largest part of its thermal insulation. The second method leads to an estimate of the fall in temperature under conditions of no wind (ΔT_v) that would produce the same value of H_n that occurs under the actual conditions; ΔT_v is influenced to only a small degree by fleece depth.

H_v at Aberdeen (Scotland) constituted 25–30% of the annual H_n in 1973. H_v persists at a high level in the summer due to the dissipation of solar heat; in the winter, H_v is associated with the enhancement of the cooling effect of low temperatures. The estimation of ΔT_v from temperature and wind alone is compared with its estimation from the combination of all factors. ΔT_v per knot of meteorological wind speed (measured at 10 m height) is ∼ 1 K when T_{a = 10}°C, with an inverse variation of ∼ 30% for 10 K.

The effect of wind can be estimated as the accumulation of heat loss during periods when heat loss exceeds 55 W m⁻², the rate that is expected at the critical air temperature. If the wind speed to which sheep were exposed in 1973 at Aberdeen had been halved, with temperature and other conditions unchanged, the year's integral of (H_{n − 55}) would have fallen from 107 to 36 MJ m⁻² for sheep with a fleece depth of 50 mm. This provides some measure of the value that can be attached to a wind break.