Erythemal UV irradiances at Lauder, New Zealand: relationship between horizontal and normal incidence.

Measurements from sensors designed to measure erythemal UV irradiance were used to relate the UV incident on a horizontal surface to that incident on a surface maintained normal to the sun throughout the day at Lauder, New Zealand. These UV measurements were also related to variations in global radiation, total column ozone and atmospheric pressure at the surface. Strong correlations were found between these variables over the 37 day observation period in the summer of 1995/1996. Results from these cross-calibrated UV sensors show that the irradiance incident on a surface normal to the sun can be significantly different from that on a horizontal surface. On clear days, the normal-incidence signal can be 30-40% greater for solar zenith angles in the range 60-70 degrees. Consequently, the risk of UV damage can be greater than reported by measurements or models that assume horizontal incidence (e.g., UV index). On cloudy days the normal-incidence UV can be less than 50% of the horizontal-incidence UV. Averaged over a day, any enhancements in normal-incidence UV over horizontal-incidence UV are smaller. The effects were strongly dependent on cloud conditions. Under clear skies the enhancements are generally less than 10%, and the integrated excess over horizontal-incidence UV is usually less than 5%. However, under cloudy skies the reductions can still be large.