Showing papers by "John L. Monteith published in 1975"

Long‐wave radiation at the ground I. Angular distribution of incoming radiation

Abstract: The apparent emissivity of the atmosphere ϵ, defined as the ratio of incoming long-wave radiation to black-body radiation at screen temperature Ta, was measured under clear skies in the English Midlands and in the Sudan. At a zenith angle Z the emissivity was given by ϵ(Z) = a + bIn(usec Z) where u is the reduced depth of precipitable water (cm). For a set of 46 scans in England, the mean values of a and b were 0·70±0·05 and 0·090 ± 0·002. Systematic deviations about these mean values could be ascribed to: (i) temperature gradients; (ii) aerosol. The Sudan measurements gave a = 0·67 ± 0·03 and b = 0·085 ± 0·002 consistent with the English results and observations already published. There is some evidence that minimum values of a have increased over the past 50 years. Integration over the hemisphere gives the flux density (Wm−2) of atmospheric radiation as 1·06 σTa4 − 119 (T in K), where σ is Stefan's constant, or 5·5 Ta + 213 (T in °C). Radiation records by Dines and Dines (1927) for overcast skies are analysed to show that the angular distribution is the same as for cloudless skies; that the mean temperature of cloud base at their site was UK below screen temperature; and that when the mean fraction of cloud cover is c, the apparent emissivity is ϵa(c) = (1 − 0·84c)ϵa(0) + 0·84c.

Transfer processes in animal coats I. Radiative transfer

Abstract: A silicon cell and a radiometer were used to measure the transmission of short wave (0.4-1 $\mu $m) and long wave (3-100 $\mu $m) radiation through the coats of sheep and cattle. The reflectivity of the same coats was measured in a spectrophotometer. Measurements of transmission and reflexion were interpreted in terms of a theory of radiation scattering developed (for vegetation) by Cowan (1971), assuming that radiation striking a single hair could be absorbed or scattered either towards or away from the skin. One of the parameters used in the theory is the fraction of radiation intercepted by a ray in unit depth of coat (p). For diffuse radiation the appropriate mean value of the interception function ($\overline{p}$) is approximately twice the value of p for a ray at normal incidence. The value of $\overline{p}$ ranged from about 9 cm$^{-1}$ for sheep's fleece to 36 cm$^{-1}$ for calf and deer coats. In the short wave spectrum, mean reflexion coefficients for the whole coat ranged from 0.30 for Welsh Mountain Sheep (black fleece) to 0.79 for Dorset Down Sheep (white fleece); corresponding values of the absorption coefficient for individual hairs were 0.02 and 0.002. On the basis of these and related figures, the absorption of solar radiation by the skin surface was evaluated for different combinations of fleece and skin colour. The combination of a light fleece and a dark skin is a trap for solar radiation because the scattering by hair is predominantly forwards.

Transfer processes in animal coats. II. Conduction and convection.

Abstract: According to reliable literature, the thermal conductivities of animal coats range from about 40 to 150 mW m$^{-1}$ K$^{-1}$ compared with 25 mW m$^{-1}$ K$^{-1}$ for still air at 20 degrees C. Greater rates of heat transfer in coats can be accounted for by (a) radiative transfer between hairs; (b) free convection induced by temperature gradients. A simple theoretical analysis of radiative transfer for the special case of a linear temperature gradient showed that in the region where boundary effects are negligible, a radiative conductivity can be estimated from 4b/3p where b is the increase of black-body radiant flux per degree Kelvin and p is the interception function defined by Cena & Monteith (1975a). Taking account of radiation, the combined molecular and radiative conductivity of coats is expected to fall between 30 and 45 mW m$^{-1}$ K$^{-1}$. Higher values, e.g. for fleece, can be accounted for by free convection. The importance of free convection in a sample of fleece with a diameter of 20 cm was demonstrated by showing that the thermal conductivity was independent of windspeed but increased with the temperature difference between the skin and the air or (T$\_{\text{s}}$ - T$\_{\text{a}}$). When the sample was held vertically, the Nusselt number for free convection was given by 1.66 (T$\_{\text{s}}$-T$\_{\text{a}}$)$^{0.7}$. The Nusselt number for a 2 cm fleece was very close to the value expected for a flat plate with the same diameter.

Transfer Processes in Animal Coats. III. Water Vapour Diffusion

Abstract: The diffusion of water vapour through samples of cured and uncured fleece and fibreglass wool was measured. The diffusion resistance of the fibreglass was close to the value expected for still air, i.e. 4.2 s cm$^{-1}$ per centimetre for samples ranging in depth from 1 to 7 cm. The resistance for natural fleece was similar to the resistance for still air up to a depth of 4 cm but at 7 cm deep was only 2.5 s cm$^{-1}$ per centimetre. The difference in behaviour of the three materials was interpreted in terms of liquid movement. By appeal to principles of similarity, an equation for sensible heat transfer by free convection from an isolated sample of fleece is used to estimate corresponding rates of latent heat transfer when the skin is wetted by sweat. When a sheep is exposed to air at a temperature close to deep body temperature the exchange of sensible heat between the skin and the air may be a trivial component of the heat balance but provided the skin is wet, the evaporative heat flux from the skin may reach 200-300 W m$^{-2}$.

I. Angular distribution of incoming radiation

Abstract: SUMMARY The apparent emissivity of the atmosphere t', defined as the ratio of incoming long-wave radiation to black-body radiation at screen temperature To, was measured under clear skies in the English Midlands and in the Sudan. At a zenith angle Z the emissivity was given by