This paper evaluates the global vitamin D effective UV (UVvitd) irradiances under cloudy conditions at a subtropical, southern hemisphere site. The UVvitd irradiances were analyzed on a horizontal plane and sampled at 5-min intervals over 18 months so that a wide range of parameters including cloud conditions, solar zenith angles (SZA) and ozone levels were taken into account. Cloud modification factors were determined from the influence of clouds on the global broadband solar radiation, and these were applied to the cloud-free vitamin D effective UV irradiance to evaluate the UVvitd irradiances on a horizontal plane for cloudy conditions. For vitamin D effective UV irradiance, cloud modification factors were found to range from 0.9 to 1.0 for no cloud and 0.4 to 0.5 for 8 octa of cloud cover. SZA played a minimal role in this variation. A comparison of the measured and calculated UVvitd irradiances for the 2004 data set in the range of SZA of 70° or less provided an R2 value of 0.90. The output of the model was compared to data measured during the first 6 months of 2005 for an SZA of 70° or less and provided an R2 value of approximately 0.82.