Some aspects of solar radiation in its relation to cattle in South Africa and Europe

Abstract

Two aspects of solar radiation in its relation to cattle in South Africa and Europe are considered. The first is the solar radiation itself and the factors influencing its intensity and total amount incident on a horizontal surface at various places. The second is the question of how much radiation is absorbed by the body surface of cattle in South Africa and Europe. A comparison of solar radiation in South Africa and Europe shows the following facts: 1. The angle of incidence of the solar rays is distinctly larger in South Africa than in Europe. 2. The midday intensities in Davos (Switzerland) are on an average nearly equal to those at Johannesburg (South Africa); at Kew (England) they are on an average lower than at Durban (S.A.). 3. The days are shorter in South Africa during summer, but longer during winter. 4. The number of hours with bright sunshine is much greater in South Africa during the whole year, particularly during winter. 5. The monthly total, amount of sun and sky radiation is equal or slightly greater in summer; it is, however, markedly greater during winter in South Africa than in Europe. 6. The yearly total amount of incident radiation is 187 Kilogram Calories per square centimetre at the South African Inland Stations as compared with 103 Cals. / sq. cm. on the lowlands of central Europe. In Part II the total absorption of radiation from the sun, the sky, and that reflected from the ground on to the body surface of cattle under South African and European conditions is calculated. The discussion is limited to a few clearly defined examples. Figures of the amount of solar and sky radiation impinging on to the animal during a clear mid-summer and a clear mid-winter day are given, the animal either standing at right angles to the solar beam or facing the sun. The absorption of the incoming radiation is determined by reflection measurements on two brown bovine hides of different breeds (Sussex x Afrikaner and high grade Afrikaner) and figures of the absorption of direct solar and sky radiation and the absorption of radiation reflected from the ground are presented. From these data the total amount of radiation absorbed by the body surface of cattle is calculated. This amount is found to be strikingly high, e.g., more than 20,000 Kilogram Calories during a clear mid-summer day, regardless whether the animal is exposed on the high veld of South Africa, in the alpine region of Switzerland or on the lowlands of central Europe. A comparison of the total amount of radiation absorbed by the hairy coat and the heat produced by metabolism shows that cattle absorb nearly three times as much heat from radiation as they produce by metabolism during an equal period. The means of losing heat in order to keep their body temperature within safe limits are discussed from a physical point of view. With regard to a possible reduction of the amount of heat which has to be eliminated from the body, the effect of shade on the amount of incident radiation is. discussed. Figures of the reduction of the incoming solar radiation by natural and artificial shade are given which show that the amount of heat absorbed by the hairy coat of cattle can, by providing shade, be reduced to 30-40 per cent. of the amount which impinges on to the animal in the open veld.