The transfer of heat by radiation and turbulence in the lower atmosphere

Abstract

The air within any element of volume may gain or lose heat by (1) absorption or emission of radiation, (2) transport of heat in the vertical by turbulence, (3) ascent of saturated air producing condensation and liberating latent heat, (4) arrival of warmer or colder air in a horizontal direction. There are not available any data in a suitable form to permit our discussing the last of these factors, but it is fairly easy to detect occasions when rapid changes are taking place by an examination of records of wind velocity and direction. The third is not operative at levels below the lowest cloud layer, and for that reason it is proposed in the present paper to restrict the discussion to the lowest layers of the atmosphere, where radiation and turbulence are the controlling factors. An endavour will be made in a subsequent paper to extend the discussion to higher levels, taking account of the third factor mentioned above. II.— Absorption and Emission in the Atmosphere . It is generally accepted that the simple gases in the atmosphere do not absorb appreciably either the shortwaved radiation from the sun or the longwaved radiation from the eath's surface and the water vapour in the atmosphere. There is some absorption by carbon dioxide, but most of the absorption in the atmosphere is produced by water vapour. There is available a series of measurements of the absorbing power of water-vapour, carried out by Hettner, and summarised by Simpson in a recent memoir. The main results of Hettner's observations may be summarised as follows: A column of air containing 0·3 mm. of precipitable water in the form of vapour will absorb completely all radiation of wave-lengths between 5·5 ·μ and 7 μ, and greater than about 14 μ, and will absorb some, but not all, of the radiation between 4 μ and 5·5 μ, between 7 μ and 8 μ, and between 11 μ and 14 μ. There is a band from 8 μ to 11 μ within which the vapour is transparent to radiation. This band is of considerable meteorological importance on account of the fact that the wave-length at which maximum intensity occurs in the spectrum of terrestrial radiation, or 10 μ, falls within it. The main absorption band of CO 2 is centred about 15 μ. Simpson states ( loc. cit. , p. 12), that the essential facts can be represented by assuming that within the restricted regions of wave-lengths concerned the emission of radiation from water-vapour is equal to black body radiation at the same temperature, and that the radiation of these wave-lengths is completely absorbed by a layer of containing 0·3 mm. of precipitable water. By the adoption of this very simple basis, Simpson was able to explain satisfactorily a number of phenomena, and it is therefore proposed to adopt the same basis in the present paper.