Abstract This research examines the influence of cloud on the cataract effective UV (UV Cat) irradiances on a horizontal plane over an extended period of 12months that included the range of cloud conditions, solar zenith angle (SZA) and ozone conditions experienced over that time. The data were collected at five minute intervals. Cloud modification factors were determined from the influence of clouds on the global broadband solar radiation and these were applied to the cloud free cataract effective UV to evaluate the UV Cat irradiances on a horizontal plane for all cloud conditions. A comparison of the measured and calculated UV Cat irradiances for the 2004 data set in the range of SZA of 70° or less provided an R 2 value of 0.85. The data in the first 6months of 2005 for an SZA of 70° or less that were at a different time to that when the technique was developed provided an R 2 value of 0.83 for the comparison of the measured and calculated UV Cat irradiances.