Modification of Global Erythemally Effective Irradiance by Clouds

Abstract

The role clouds play in the modification of global radiation is still a major uncertainty in the risk assessment of UV effects on ecological systems and human health. This study presents cloud transmission data obtained from measurements with Robertson-Berger meters and simultaneous cloud observations. The global transmission of erythemally weighted 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 significant difference between the attenuation of erythemal and total global radiation. The large variance of data results from lumping together data from different cloud types. Classification of data according to cloud forms yields a more satisfactory 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 description of the cloud influence on irradiance and therefore a more accurate risk assessment.

UV and global solar radiation in Łódź, Central Poland

“…This usually lasted for duration of less than an hour, most frequently of 10-20 min. Clouds generally reduce the incoming solar radiation flux to the Earth's surface as the solar disk is obscured, but they may also enhance solar irradiance, especially UV, when the solar disk is partly covered by clouds (Thiel et al, 1997;Degünther and Meerkötter, 2000). Values of 5-15% are commonly cited in the literature (Monteith and Unsworth, 1988;Robinson, 1977).…”

Cloud observations for the statistical evaluation of the UV index at Toowoomba, Australia

“…Previous research, based on manual observations, has established that for cloud fractions less than 0.5 (4 okta), the attenuation of UV by cloud is independent of the cloud type, whereas for cloud fractions of 0.87 or greater (7 and 8 okta), there is a large difference in the attenuation of UV by high level clouds and by cumulus clouds (Thiel et al 1997). …”

Cloud cover and horizontal plane eye damaging solar UV exposures

“…Previous research has reported on models to predict the global erythemal UV based on the cloud amount and cloud type (for example, Thiel et al 1997). Models have been developed for evaluating the global UVB based on a range of cloud parameters (Sabburg and Wong 2000b) and the diffuse solar UVA and UVB for partly cloudy skies (Grant and Gao 2003).…”