Horizontal and sun-normal spectral biologically effective ultraviolet irradiances

The dependence of the spectral biologically effective solar UV irradiance o n the orientation of the receiver with respect to the sun has been determin ed for relatively cloud-free days at a sub-tropical Southern Hemisphere lat itude for the solar zenith angle range 35-64 degrees. For the UV and biolog ically effective irradiances, the sun-normal to horizontal ratio for the to tal UV ranges from 1.18 +/- 0.05 to 1.27 +/- 0.06. The sun-normal to horizo ntal ratio for biologically effective irradiance is dependent on the relati ve effectiveness of the relevant action spectrum in the UV-A waveband. In c ontrast to the total UV, the diffuse UV and diffuse biologically effective irradiances are reduced in a sun-normal compared with a horizontal orientat ion by a factor ranging from 0.70 +/- 0.05 to 0.76 +/- 0.03. (C)1999 Elsevi er Science S.A. All rights reserved.