Elements of radio meteorology: how weather and climate cause unorthodox radar vision beyond the geometrical horizon

Abstract

Radio refraction in the lower atmosphere often makes it possible for radar receivers to see round the curved surface of the earth and to plot targets far below the geometrical horizon. This phenomenon of super-refraction is associated with the occurrence of a radio duct, or wave guide, close to the surface of the earth. Any two points within such a duct are intervisible by refraction even though each is below the other's geometrical horizon. Field strength at one due to a transmitter at the other can therefore be of the same order of magnitude as would occur in complete free space, provided that the wavelength is sufficiently short and atmospheric attenuation is unimportant. With increase of wavelength, however, the duct (being qualitatively similar to a wave guide in the form of a metallic pipe) ceases to be able to guide the waves efficiently: for sufficiently long wavelengths propagation takes place substantially as though no duct existed. Dekametre and longer wavelengths are largely free from super-refraction because ducts sufficiently large to guide these wavelengths efficiently do not occur in the atmosphere. Centimetre waves are prone to super-refraction because ducts extending from the earth's surface up to a height of the order of 100 ft are sufficient to guide them quite efficiently and are of frequent occurrence. In its most intense form (ducts extending from the surface up to heights of a few thousand feet) efficient guiding is possible, not merely for centimetre wavelengths, but also for decimetre and metre wavelengths. In its weak form, super-refraction merely causes a decrease in attenuation beyond the horizon, particularly at centimetre wavelengths.Super-refraction is essentially a fine-weather phenomenon, and therefore tends to occur in an intense form in tropical (but not equatorial) climates. Super-refraction occurs when the upper air is unusually warm and dry in comparison with air at the surface of the earth, the associated unusual gradients of temperature and humidity being the cause of the downward refraction. Inland during fine weather, superrefraction is most noticeable at night and usually disappears during the warmest part of the day. Over the sea, super-refraction is most marked where the warm dry air of an adjacent land-mass is able to extend out over a comparatively cool sea. Only rather qualitative forecasting of super-refraction is possible with present meteorological knowledge.