Long-wave radiation at the ground

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

Some aerodynamic features of a cotton canopy

Analysis of profiles measured over irrigated cotton in the Sudan Gezira showed that the crop boundary layer remained stable throughout most of the day. The shape of wind profiles measured within the canopy suggested that momentum was absorbed mostly in the upper layers of the canopy, with the lower regions remaining isolated from the microclimate above. A similarity analysis based on dynamic scaling factors yielded a generalized wind profile from which momentum diffusivities and mixing lengths were calculated. The analysis showed that aerodynamic features of the upper layers of the canopy were characterized by the friction velocity and the height of the zero-plane displacement

Spectral distribution of solar radiation. I: direct radiation

Measurements of the spectral distribution of direct solar radiation, made in both wide and narrow wavebands, are reported. The measurements suggest that the wavelength dependence of aerosol attenuation in central England, in summer, can generally be described by a power law with wavelength exponent of about 1.7. Attenuation of visible radiation is reported in terms of an integral turbidity coefficient, τv, and is shown to depend to some extent on air mass origin, air masses from the continent being more turbid than maritime air masses. The mean value of τv a is 0.31 with a range of 0.05 to 0.6. The ratio of visible to total direct irradiance is found to be a linear function of τv, the constants of linearity being functions of zenith angle. From these relationships a semi‐empirical model is developed enabling spectral irradiance, visible irradiance and total solar irradiance to be estimated from arbitrary values of turbidity and zenith angle

Longwave radiation at the ground: IV. Comparison of measurement and calculation of radiation from cloudless skies

On cloudless days in summer, longwave irradiance was measured with a radiometer and was calculated from a radiation chart using records from radiosondes released at regular intervals. The results show that the chart underestimated the longwave flux density at the ground by up to 40W/m^(-2), corresponding to an extra atmospheric emissivity of up to 0.12. The extra emissivity was correlated with turbidity during the day, and on turbid days it showed a diurnal variation with a maximum at noon. It is suggested that emission in the atmospheric window from dust, pollen and spores may account for some of the observed excess

Turbulent transfer of sulphur dioxide to a wheat crop

Fluxes of sulphur dioxide to wheat were measured over two growing seasons. The deposition velocity, vg, depended primarily on affinities of surfaces for SO2 and varied from less than 0.1 to about 1.5cms−1. Stomata were an important sink, and diurnal changes in stomatal resistance accounted for most of the variation in vg when leaves were green and dry. When leaves were wet with dew, vg was generally large, irrespective of the physiological state of vegetation. A resistance analogue of transfer to the canopy agrees well with published work and is used to estimate seasonal uptake of SO2 by the crop, showing that 30% of the sulphur content of the crop at harvest was probably supplied by absorption of SO2 through stomata

The angular distribution and interception of diffuse solar radiation below overcast skies

The angular distribution of diffuse radiation was measured from May 1976 to May 1977 with a set of purpose-built actinometers. Radiance distributions for 98 overcast hours were fitted well by an expression similar to the conventional function for a 'standard overcast sky' (SOC). However, values of the coefficient b averaged 1.23, significantly smaller than for the SOC (b = 2). Expressions for the interception of diffuse irradiance by sloping planes are discussed

The diffuse solar irradiance of slopes under cloudless skies

The diffuse irradiance of slopes relative to the diffuse irradiance of a horizontal surface is calculated by integrating the mean radiance distributions of cloudless skies in Britain for a wide range of solar elevations, azimuths and slope angles. There is reasonable agreement with measurements reported from a number of stations; discrepancies are probably due to errors in measurements and to the influence of aerosol on the radiance distributions. A simple model of the diffuse irradiance of slopes, which takes account of circumsolar radiation, is proposed, and agrees well with integrated values of irradiance

Shade-ring corrections for pyranometer measurements of diffuse solar radiation from cloudless skies

When a shade ring is used to shield a pyranometer from direct solar radiation, a correction to the measured diffuse radiation is necessary to account for diffuse radiation intercepted by the ring. A general analysis is developed to relate shade-ring corrections to the radiance distribution of diffuse radiation. The corrections are split into two components: a geometric component based on an isotropic sky and varying with shade-ring dimensions; and an anisotropy component, relatively independent of ring dimensions. Shade-ring corrections are calculated using mean distributions of the radiance of cloudless skies. Tables and figures are given for calculating these corrections as functions of latitude, ring dimensions and time of year. Comparisons with published measurements at a variety of sites reveal generally good agreement with calculations. Some of the discrepancies may be due to differences in aerosol scattering

Aerosol and solar radiation in Britain

The irradiance of the solar beam was measured on cloudless days at Sutton Bonington in the English Midlands and at sites in north-west Scotland. Total and diffuse fluxes were also measured on some days. An attenuation coefficient for aerosol Ta was defined by S(Ta) = S(o) exp ( - Tam) relating the measured flux at normal incidence S(Ta) to the flux calculated for a. dust-free atmosphere when the air mass number is m. Changes of Ta from day to day were related to changes of air mass origin; local sources of aerosol were relatively unimportant. In maritime air, Ta ranged from 0·05 to 0·15, and in continental air, from 0·l to 0•5. In a tropical maritime air mass, Ta decreased from 0·13 at sea level to 0·07 at 1,340 m. The fraction of (ultra-violet + visible) to total radiation was (0·54 - 0·28 -ra) and the ratio of diffuse to total radiation (m < 2) was (0·l + 0·7 Ta), The ratio of total scattering to absorption by aerosol decreased from 4 at m = 1 ·1 to 0·5 at m = 2. Mean monthly values of Ta at four Meteorological Office stations were calculated from records of solar radiation and hours of sunshine and corresponding values of total and diffuse flux were tabulated for ' isolated ', ' rural ' and ' urban ' sites

Attenuation of total solar radiation by aerosol over Britain and the Atlantic Ocean

In a recent note, Collier and Lockwood (1974) compared measurements of total solar radiation on cloudless days at an inland site in England with radiation calculated from empirical expressions derived from (i) data from weather ships in the eastern Atlantic (Lumb 1964) and (ii) measurements of radiation and turbidity in central England (Unsworth and Monteith 1972). Collier and Lockwood concluded that radiation received inland under cloudless skies was only about 60% of that received over the sea. As this figure is inconsistent with the whole literature of radiation climatology in Britain, we believe there must be large errors both in the measurements they recorded and in estimates they derived from independent empirical relationships. Collier and Lockwood tabulated hourly mean values of total solar radiation S, for three cloudless days (20 August 1971, 14 October 1971 and 21 January 1972) and they plotted the dependence of S, on solar elevation O for these and other days. Hourly means of S, are recorded by the Meteorological Office and other establishments for a number of sites, and comparison shows clearly that for given values of 0, under cloudless skies, Collier and Lockwood's values are 30-40 % lower than those recorded elsewhere in Britain