Showing papers by "John L. Monteith published in 1962"

Radiative temperature in the heat balance of natural surfaces

Abstract: A Linke-Feussner radiometer was used to measure the radiative temperature of natural surfaces, with small errors caused by the divergence of radiative flux and by its angular variation. On typical cloudless summer days, the maximum temperature of tall crops and of open water was close to maximum air temperature; a bare soil surface exceeded air temperature by 20°C; short grass was the coldest surface at night. A linear variation of net (total) radiation with net short-wave radiation was established for several tall crops, and the increase in long-wave loss per unit increase of net radiation (heating coefficient) was 0·08. Daily totals of net radiation as a percentage of solar radiation income were: 37 per cent (bare soil), 41 per cent (short grass), 46 per cent (tall crop), and 53 per cent (water). When the aerodynamic character of a crop is known, the effective resistance of the stomata to water-vapour diffusion can be related theoretically to the difference between surface and air temperature. For rough vegetation, estimated stomatal resistance is 0·4 to 0·8 sec cm−1 and the estimated heating coefficient is almost independent of wind speed at about 0·1.

Attenuation of solar radiation: A climatological study

Abstract: Daily totals of direct and diffuse radiation transmitted by a cloudless atmosphere are calculated from the absorption and scattering coefficients given by Houghton (1954). Estimates of total (direct plus diffuse) radiation (T1) agree well with extreme maxima recorded at several British stations, but during spells of cloudless summer weather in south-east England attenuation by aerosol decreases total radiation on average by 10 per cent, and increases the ratio of diffuse to total radiation by about 8 per cent of T1. The ratio of mean monthly radiation at country stations T2 to estimated T1 can be expressed as a function of cloud reflection (ρ), cloud absorption (ϕ), and surface reflection (α); and the values ρ = 0·50, ϕ = 0·16 are consistent with surface and aerial measurements. With α = 0·20, T2/T1 is approximately (1−0·61c) (0·6 < c < 0·9) where c is fractional cloudiness. The diffuse component beneath cloud can be estimated separately. Absorption by man-made aerosol reaches maxima of 26 per cent at Kew and 30 per cent at Kingsway in January, and varies seasonally with measured smoke. In the country, absorption by pollution is negligible in summer but may reach 5 to 10 per cent in winter. At Kew, absorption by cloud and total back-scattering (local planetary albedo) are about 9 per cent and 41 per cent of extra-terrestrial flux respectively, with little seasonal variation.

Simple devices for radiation measurement and integration

Abstract: Simple thermopiles formed by copper-plating a constantan wire were used to build a solarimeter and a net radiometer. Output was 5–10 mV per cal·cm−2·min−1, adequate to operate a small portable galvanometer for spot readings, a continuously recording potentiometer, or, for integration, a Siemens electrolytic meter. The net radiometer was unventilated, but when oscillated manually, output was independent of wind speed. With two solarimeters in a null-balance circuit, reflection and transmission coefficients of a field crop were read directly on a dial. A circuit was designed to integrate solarimeter output and the standard error of weekly readings was only ±40 cal·cm−2.