Measurements of erythemally weighted UV radiation during about 600 days at different sites in Bolivia and Germany ranging from 550 to 5240 m above sea level have been used to derive the altitude effect AE under cloud-free conditions. In Germany, AE values between 7 and 16%/km have been obtained. In Bolivia, the altitude effect between the lowlands and the Bolivian plateau reached values of 5--10%/km. An altitude effect of 8--23%/km has been measured between the plateau and a high-mountain station. In accordance with previous studies these results indicate that the altitude effect of UV irradiance cannot be described by a single number in %/km, because it strongly depends on the atmospheric and surface parameters. In order to understand the high variability of the AE, the effects due to variations in solar elevation, albedo, and aerosol properties on UV radiation and the AE have been analyzed. To eliminate the influence of clouds, an algorithm for the selection of cloud-free time intervals has been developed and applied. Furthermore, the measured data have been normalized to a fixed ozone content to avoid masking of the AE by different ozone amount. In addition, the background altitude effect, i.e., the AE resulting only from the reduced barometric pressure and reduced ozone content with increasing altitude, has been modeled. Depending on solar elevation and albedo, it ranges between 3 and 7%/km. Measured higher values of the AE, as well as negative values of the AE, are explained by the specific regional aerosol conditions, with important sources at high altitudes. The aerosol influence on UV is shown in detail for extreme conditions after strong bonfires in connection with a local holiday.