Showing papers by "John L. Monteith published in 1972"
TL;DR: Conventional estimates of efficiency in terms of the amount of solar radiation incident at the earth's surface provide ecologists and agronomists with a method for comparing plant productivity under different systems of land use and management and in different * Opening paper read at IBP/UNESCO Meeting on Productivity of Tropical Ecosystems.
Abstract: In thermodynamic terms, ecosystems are machines supplied with energy from an external source, usually the sun. When the input of energy to an ecosystem is exactly equal to its total output of energy, the state of equilibrium which exists is a special case of the First Law of Thermodynamics. The Second Law is relevant too. It implies that in every spontaneous process, physical or chemical, the production of 'useful' energy, which could be harnessed in a form such as mechanical work, must be accompanied by a simultaneous 'waste' of heat. No biological system can break or evade this law. The heat produced by a respiring cell is an inescapable component of cellular metabolism, the cost which Nature has to pay for creating biological order out of physical chaos in the environment of plants and animals. Dividing the useful energy of a thermodynamic process by the total energy involved gives a figure for the efficiency of the process, and this procedure has been widely used to analyse the flow of energy in ecosystems. For example, the efficiency with which a stand of plants produces dry matter by photosynthesis can be defined as the ratio of chemical energy stored in the assimilates to radiant energy absorbed by foliage during the period of assimilation. The choice of absorbed energy as a base for calculating efficiency is convenient but arbitrary. To derive an efficiency depending on the environment of a particular site as well as oil the nature of the vegetation, dry matter production can be related to the receipt of solar energy at the top of the earth's atmosphere. This exercise was attempted by Professor William Thomson, later Lord Kelvin, in 1852. 'The author estimates the mechanical value of the solar heat which, were none of it absorbed by the atmosphere, would fall annually on each square foot of land, at 530 000 000 foot pounds; and infers that probably a good deal more, 1/1000 of the solar heat, which actually falls on growing plants, is converted into mechanical effect.' Outside the earth's atmosphere, a surface kept at right angles to the sun's rays receives energy at a mean rate of 1360 W m-2 or 1f36 kJ m-2 s-1, a figure known as the solar constant. As the energy stored by plants is about 17 kJ per gram of dry matter, the solar constant is equivalent to the production of dry matter at a rate of about 1 g m-2 every 12 s, 7 kg m-2 per day, or 2 6 t m-2 year-'. The annual yield of agricultural crops ranges from a maximum of 30-60 t ha-' in field experiments to less than I t ha-' in some forms of subsistence farming. When these rates are expressed as a fraction of the integrated solar constant, the efficiencies of agricultural systems lie between 0-2 and 0 004%, a range including Kelvin's estimate of 0-1%. Conventional estimates of efficiency in terms of the amount of solar radiation incident at the earth's surface provide ecologists and agronomists with a method for comparing plant productivity under different systems of land use and management and in different * Opening paper read at IBP/UNESCO Meeting on Productivity of Tropical Ecosystems, Makerere University, Uganda, September 1970.
2,278 citations
TL;DR: In this article, the authors measured the irradiance of the solar beam on cloudless days at Sutton Bonington in the English Midlands and at sites in north-west Scotland and calculated the mean monthly values of T. 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.
Abstract: SUMMARY 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 T. was defined by S(T~) = S(o) exp ( - Ta m) relating the measured flux at normal incidence S(T.) to the Rux calculated for adust-free atmosphere when the air mass number is m. Changes of T. from day to day were related to changes of air mass origin; local sources of aerosol were relatively unimportant. In maritime air, ra ranged from 0.05 to 0.15, and in continental air, from 0.1 to 0.5. In a tropical maritime air mass, T~ decreased from 0.13 at sea level to 0.07 at 1,340 m. The fraction of (ultra-violet f visible) to total radiation was (0.54 - 0.28 7.) and the ratio of diffuse to total radiation (m < 2) was (0.1 + 0.7 7.). 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 T. 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. The presence of solid particles in the Earth’s atmosphere has important consequences for the transmission of solar radiation and for the nature of the radiation rkgime at the ground. The absorption of solar energy by a layer of aerosol increases the radiative heating of the atmosphere and decreases the amount of energy available at the surface. Scattering by aerosol increases the amount of radiation which is reflected by the atmosphere into space and increases the downward flux of diffuse radiation at the Earth’s surface. Attenuation also produces changes in the spectral composition of solar radiation which are significant biologically. To estimate the amount of radiant flux which is absorbed and scattered by aerosol as distinct from other atmospheric constituents, measurements of direct and diffuse radiation at, the ground may be compared with the fluxes predicted below a model atmosphere containing appropriate amounts of ozone, water vapour, and carbon dioxide (G. D. Robinson 1962, 1966). The height distribution of aerosol can be inferred by measuring solar radiation from aircraft (Roach 1961), and the presence of particles as high as 50 km has been demonstrated by measuring the scattering of light from searchlight beams (Elterman, Wexler and Chang 1969). Concern about possible changes in global climate has stimulated new interest in the radiative effects of aerosol. Recent calculations by Rasool andSchneider (1971) imply that any future increases of aerosol content will decrease the mean surface temperature of the Earth and that the heat balance of the atmosphere may become increasingly sensitive to changes of aerosol content. To be able to detect the radiative effects of changing aerosol content, it is essential to establish baselines for the income of solar radiation in different parts of the world and to show how this income is related to aerosol load. This kind of exercise has been attempted at relatively few sites. Valko (1963) analysed turbidity measurements at LocarnoMonti in Switzerland and demonstrated marked daily and annual changes in the strength of the direct solar beam which he ascribed to differences in the composition of aerosol in different air masses. Flowers, McCormick and Kurfis (1969) reported measurements of turbidity from a network of stations in the USA equipped with sun photometers. These instruments record the irradiance of the direct beam at a wavelength of 0.55 pm but they
138 citations
TL;DR: The transfer of latent heat from an animal to its environment is often estimated by multiplying the loss in weight attributable to evaporation by the latent heat of vaporization of water, λ, which decreases from 2,501 J/g at 0° C to 2,406 J/G at 40° C.
Abstract: THE evaporation of water from the skin and the respiratory tract is an important mechanism of heat loss in homeotherms, and man's ability to sweat is essential for his survival when the temperature of the environment approaches or exceeds body temperature. The transfer of latent heat from an animal to its environment is often estimated by multiplying the loss in weight attributable to evaporation by the latent heat of vaporization of water, λ, which decreases from 2,501 J/g at 0° C to 2,406 J/g at 40° C.
23 citations