MODIFICATION OF GLOBAL ERYTHEMALLY EFFECTIVE IRRADIANCE BY CLOUDS

The role clouds play in the modification of global radiation is still. a major uncertainty in the risk assessment of UV effects on ecologica l systems and human health. This study presents cloud transmission dat a obtained from measurements with Robertson-Berger meters and simultan eous cloud observations. The global transmission of erythemally weight ed irradiance depends strongly on cloud amount and can be described by a cubic function. The comparison with results derived from long-term records of total global irradiance indicates no statistically signific ant difference between the attenuation of erythemal and total global r adiation. The large variance of data results from lumping together dat a from different cloud types. Classification of data according to clou d forms yields a more statisfactory fit. The coefficient of the cubic term characterizes the ability of various cloud forms to attenuate UV radiation. It varies between 0.4 for high clouds and approximately 1.0 for cumulonimbus. This attenuation parameter allows a quantitative de scription of the cloud influence on irradiance and therefore a more ac curate risk assessment.