Theor. Appl. Climatol. 75, 105–116 (2003) DOI 10.1007/s00704-003-0729-5 1 Instituto de Fı ´sica Rosario, Universidad Nacional de Rosario, Argentina 2 Institute of Medical Physics, University of Innsbruck, Austria Effect of clouds on UV and total irradiance at Paradise Bay, Antarctic Peninsula, from a summer 2000 campaign E. Luccini 1 , A. Cede 2 , and R. D. Piacentini 1 With 6 Figures Received August 9, 2002; revised January 4, 2003; accepted January 28, 2003 Published online May 20, 2003 # Springer-Verlag 2003 Summary The analysis of ground-based measurements of solar erythemal ultraviolet (UV) irradiance with a Solar Light 501 biometer, and total (300–3000 nm) irradiance with an Eppley B&W pyranometer at the Argentine Antarctic Base ‘‘Almirante Brown’’, Paradise Bay (64.9  S, 62.9  W, 10 m a.s.l.) is presented. Measurement period extends from February 16 to March 28 2000. A relatively high mean albedo and a very clean atmosphere characterise the place. Sky conditions were of generally high cloud cover percent- age. Clear-sky irradiance for each day was estimated with model calculations, and the effect of the cloudiness was studied through the ratio of measured to clear-sky value (r). Two particular cases were analysed: overcast sky with- out precipitation and overcast sky with rain or slight snow- fall, the last one presenting frequently dense fog. Total irradiance was more attenuated than UV by the homogene- ous cloudiness, obtaining mean r values of 0.54 for erythe- mal irradiance and 0.30 for total irradiance in the first case (without precipitation) and 0.27 and 0.17 respectively in the second case (with precipitation). Mean r values for the complete period were 0.58 for erythemal irradiance and 0.43 for total irradiance. Erythemal and total daily insola- tions reduce quickly at this epoch due to the increase of the noon solar zenith angle and the decrease of daylight time. Additionally, they were strongly modulated by cloudiness. Measured maxima were 2.71 kJ=m 2 and 18.42 MJ=m 2 re- spectively. Measurements were compared with satellite data. TOMS-inferred erythemal daily insolation shows the typical underestimation with respect to ground measure- ments at regions of high mean albedo. Measured mean total daily insolation agrees with climatological satellite data for the months of the campaign. 1. Introduction Solar radiation plays a fundamental role in the development of the ecosystems. In the delicate Antarctic environment, total radiation is very rel- evant as the different wavelength ranges that compose it cause a variety of effects, many of them also interrelated. Ultraviolet (UV: 280– 400 nm) has sufficient energy to produce damage at molecular and biological level (Jagger, 1985; H€ ader, 1997; Whitehead et al., 2000), photosyn- thetically available radiation (PAR: 400–700 nm) is determinant for photosynthesis and ocean pri- mary productivity (Baker, 1997; Whitmarsh and Govindjee, 2003) and infrared (IR: > 700 nm) is important for the radiative budget, which influ- ence e.g. on the polar ice melting (Bintanja and van den Broeke, 1996; Bintanja, 2000; Wendler et al., 2000). The worldwide ozone depletion detected in the past decades, with the critical phenomenon of ‘‘ozone hole’’ during the austral spring (Bojkov and Fioletov, 1995; WMO, 1999), motivated major efforts to measure the UV levels in the Antarctic region and to evaluate the consequences of its eventual increase (Weiler and Penhale, 1994; Neale et al., 1998). Geographic and atmospheric characteristics make irradiance conditions at Polar Regions very