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Abstract— The UV spectral irradiance on horizontal and vertically oriented surfaces was measured throughout a cloudless day (18 July 1995) at Izana station, Tenerife, using a Bentham DTM300 spectroradiometer scanning from 290 to 500 nm in steps of 5 nm. Results show that irradiance measured on a horizontal surface is not proportional to irradiance on a vertical surface. The relation between the two depends upon orientation of the vertical surface, zenith angle and wavelength. At short UVB wavelengths surfaces directed toward the solar azimuth received their maximum irradiances much closer to solar noon than the maxima for longer wavelengths. Some vertical surfaces also received significantly more irradiance than the horizontal surface at long wavelengths during all but the central hours of the day, while at short wavelengths all vertical irradiances were less than the horizontal except for the measurements at the extreme ends of the day. Erythemally effective radiation followed the diurnal pattern of irradiations for short UVB wavelengths.