The 10 min sums of erythemal UV-B irradiances measured during approximately 10 months at two locations with different altitudes were analyzed. The magnitude of the altitude effect averaged over the whole period varies between 4 and 8%/1000 m. The value of radiation amplification factor (RAF) derived from the relationship between UV-B irradiance and total ozone is about 1.1. The value of RAF is slightly higher if derived from the relationship between the ratio of UV-B to global solar irradiance and total ozone. A statistical model relating clear-sky UV-B irradiance with solar zenith angle, total ozone, and Julian day was developed. Unfortunately, knowledge of the daily average total column ozone does not adequately improve the forecast of UV-B irradiance. ¿ 2000 American Geophysical Union