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.

Solar ultraviolet spectroradiometry in New Zealand: instrumentation and sample results from 1990.

In 1988 the New Zealand Department of Scientific and Industrial Research initiated a program to characterize the spectrum of solar ultraviolet radiation reaching the ground in New Zealand and to identify the extent and causes of its variability. Routine measurements began at Lauder (45 degrees S 170 degrees E) in December 1989. The instrumentation, measurement strategy, and calibration procedures are discussed and uncertainties in the measurements are analyzed. With the present system useful measurements at 1-nm resolution are limited to irradiances greater than 10(-3) microW cm(-2) nm(-1), which corresponds to a lower limit in wavelength in the region 290-295 nm (depending on the Sun angle and ozone amount). This is a useful lower limit for many applications of relevance to the biosphere. Results from the first year of operation are presented and discussed.