An assessment of the global UV solar radiation under various sky conditions in Maceió-Northeastern Brazil

The first observations of UV (290–400 nm) ever made at an agro-meteorological/radiometric station located in the metropolitan region of Maceió (9°28′S, 35°49′W, 127 m), Northeast Brazil, during 2008 are reported in this work. Solar UV radiation variability in different time scales and its fractional composition in the global solar radiation (G), that is, FUV = UV/G is investigated under different sky conditions. The maximum daily UV solar irradiation (HUVd) was 0.85 MJ m−2 and the annual average 0.55 MJ m−2. It was noticed that the largest contribution to the annual variation of the daily totals of UV are attributed to cloudiness than seasonality. Under partially cloudy sky, multi reflection was responsible for an increase of 11.1 and 18.8% in the UV levels, relatively to clear days. FUV is inversely proportional to the global atmospheric transmittance (KT); the daily percentage variation of FUV was in the range of 2.33–2.95% in clear days and from 2.60 to 3.59% in cloudy days, with the annual daily average value of 2.8%. Linear empirical relations between the UV and G components for hourly and daily intervals under different KT intervals were established and recommended, with the coefficient of determination (R2) generally above 0.91. -- Highlights: ► UV and global solar radiation are evaluated in various sky conditions. ► Largest variations of the daily totals of UV are attributed to cloudiness. ► The phenomenon of multi reflection is responsible for rapid increases in UV levels. ► The UV fraction is inversely proportional to the global atmospheric transmittance.