The absorption of electromagnetic waves in the atmospheric gases, and their absorption and scattering by various forms of precipitation, are discussed from the aspect of their effects on the performance of centimetre-wave radar equipment. Particular attention has been devoted to the phenomena to be expected at the wavelength of 3.2 cm, since this is the wavelength now used, and likely to be more widely used, for marine navigational radar.At a wavelength of 3.2 cm, absorption by the atmospheric gases is hardly significant, and absorption and scattering in rain are likely to cause most of the appreciable reductions in the range of detection. Whilst it is not possible to make a complete generalization, it appears that, at this wavelength, the deterioration in radar performance on targets having echoing areas greater than, say, 2 000 m2(e.g. ships of more than about 10 000 tons) arises mainly from attenuation produced in rain rather than as a result of masking by echoes from the precipitation itself. With smaller targets, however, this masking will often be the more serious factor. The heavy rates of rainfall common in tropical-equatorial regions will produce a much greater deterioration in radar performance than will the rainfall likely to occur at all frequently in temperate regions.Although very intense snow-storms can produce echoes sufficiently strong to be troublesome, the rate of precipitation required is such that its frequency of occurrence is unlikely to be great. Attenuation occurring in snowfalls is generally of little importance. In dense fogs reductions in detection range may be appreciable—especially in polar regions—when the optical visibility falls below about 25 m; in this case, attenuation is the significant factor.