The most extensive study of frequency, magnitude and sky properties associated with UVB enhancement is reported. The first integrated automatic sky camera and irradiance measurement system was used for simultaneous data collection over the solar zenith angle range of 4.2 to 64.3¿. It is reported that 3% of UVB irradiance measurements collected at intervals of 6 minutes, over a one-year period, were cloud enhanced. It was found that 85% of possible UVB enhancements occurred for solar zenith angles ranging from 40 to 63¿. The study was undertaken during September 1997 to August 1998 at Toowoomba, Australia (27.6 ¿S, 151.9 ¿E). The maximum UVB enhancement was found to have an irradiance of 8% greater than that of an equivalent clear-sky. A mechanism for UVB enhancement for cases of solar obstruction by cirrus cloud is postulated. Analysis showed that 86% of enhancement occurred under hazy/cirrus skies. ¿ 2000 American Geophysical Union