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Refractive-index variations in the atmosphere cause errors in radar measurements. This paper presents the results of a theoretical study of the elevation-angle error due to the refraction of electromagnetic waves in the troposphere. The study is based upon ray theory, using standard meteorological data reported by the United States Weather Bureau for the surface and the standard pressure levels. Computed errors are tabulated for thirty-four monthly mean refractive-index profiles selected as being representative of various type air masses for different seasons and latitudes. In order to get an indication of the spread in propagation errors to be expected during a particular season for fixed locations, computations have been carded out based on the 0300Z soundings for the odd days of January (1950-1954) and July (1950-1953) for nine U. S. locations. The diurnal influence is investigated by analyzing the variations in surface refractive index for January and July, 1953, and also for nine U. S. locations.