Some problems in radio meteorology

Refraction of radio waves of wavelength less than about ten metres in the troposphere is a phenomenon usually associated with quite steep inversions of temperature and lapses of humidity within the first few thousand feet of the atmosphere and often within the first hundred feet. A radio ray may be bent downwards with a curvature exceeding the curvature of the earth either by an inversion of temperature exceeding about 5°F./100 ft. or by a lapse of humidity exceeding about 1/2 gm./kg./100 ft., or by a combination of the two. To cause marked unorthodox radio propagation these steep gradients need to be maintained through a layer of atmosphere whose thickness varies from say 50 ft. to 500 ft. depending on the radio wavelength under consideration. Generally speaking, the lower in the atmosphere the gradients occur the more striking is the phenomenon or radio refraction, and steep gradients at heights above about 5,000 ft. are not usually of great interest. The obvious meteorological phenomena likely to cause radio refraction are nocturnal radiation‐inversions over land, advection‐inversions caused by warm dry air over land moving out over sea, and low subsidence‐inversions. High subsidence‐inversions are not usually a direct cause of radio refraction; their presence may however indicate weather conditions favourable to development of radiation or advection‐inversions. The meteorological problem is to specify the profiles of temperature and humidity associated with these phenomena at least with sufficient accuracy to state whether the above‐mentioned gradients are exceeded, and if so between what intervals of height. Moreover it is necessary to be able to do this not merely in a special experiment but also from the ordinary data available in synoptic meteorology. It is suggested that the way to do this is to formulate a satisfactory theory of how the profiles of temperature and humidity arise, and then to check the theory carefully by experiment. The upshot should be a means of specifying the profiles in terms of parameters measuring the stability of the air, the windspeed, and so on, and these should be ascertainable from ordinary meteorological data. The application of the conjugate power‐law theory of eddydiffusion to the problem is described, as well as a comparison which had been made with observed profiles of temperature and humidity. The comparison, although satisfactory in some respects, seems to indicate that the profile‐index involved in the theory shows more signs of depending on the degree to which the diffusion‐process has developed than on the stability of the air. It is suggested that what is required is a theory in which the coefficient of eddy‐diffusion increases linearly from zero at ground level to a maximum at no great height, and then decreases again.