UV Enhancement by Cloud

Abstract

In Chapter 4 the effect of clouds on solar U V radiation has been studied. Figure 4–1 illustrated the effect of clouds on UV radiation during 1 year of continuous daily erythemal UV measurements at Toowoomba, Australia (27.6°S, 151.9°E, 693 m above sea level). The effect of clouds could be seen by the general decrease of the UV levels from the clear-sky levels (i.e. the maximum envelope of the data points). It has also been mentioned that since the 1960s the measurement of UV radiation at the surface of the earth during cloudy conditions, has sometimes revealed levels exceeding equivalent clear-sky values under cloudy conditions. This is commonly referred to as cloud enhanced UV, the subject of this chapter, and has been reported over different timescales and for different wavelengths. Figure 5–1 illustrates possible cases of UV enhancement on a daily basis, i.e. those data points shown above the equivalent modeled clear-sky values. Sometimes these data points are not confirmed to be actual cases of enhancement, e.g. Bais et al. (1993) found a slight increase of the UV irradiance during partly cloudy conditions compared to clear-sky conditions, however, they could not confirm if this was actual enhancement as the increase was within the instrument’s uncertainty limits.