Spectrally resolved measurements of solar ultraviolet (UV) radiation were made at adjacent mountain top and valley sites, vertically separated by 1 km. Data was collected over a period of 4 days in August 1993 providing simultaneously measured spectra for a range of zenith angles with both sites in direct sunlight and less than 2 octas of cloud cover. As there was no snow cover the local albedo was similar at both sites. Irradiances at the mountain site were 9% higher than the valley at 370 nm, increasing gradually to 11% at 320 nm. Across the UVB waveband the altitude effect increased dramatically to 24% at 300 nm. The magnitude of the altitude effect was independent of zenith angle, within the measurement accuracy, for the range z=30¿--70¿. These measured differences between the mountain and valley station can be interpreted by model calculations using the corresponding differences in air pressure, column ozone and aerosol content. ¿ American Geophysical Union 1994