Ground-based cloud coverage remote detection and monitoring: major drawbacks and solutions

The modelling of the solar irradiance at ground, as known, applies the Lambertlawfortheattenuationofdirectradiationandtheisotropicscatteringfordiffused radiation from other directions. These conditions in field measuring are not frequent for severalreasons.Firstofall,somesubstances,presentintheatmosphere(ozoneorsulphur dioxidefortheUVbands)absorb theradiationindifferentialway.Someelements, liquid or solid, diffuse in non-isotropic way the radiation for not homogeneous distribution of the atmosphere. The clouds are the most important factor for these effects. Therefore, the knowledge of the sky situation is a parameter very useful to solar irradiance at ground studies. Within this context, the paper aims at showing the results of campaigns carried out in polarregions, such as Terra NovaBay (Antarctica, 74.07 � S-164.083 � E) andNy Alesund (Svalbard islands, 78.917 � N-11.933 � E), where automatic all-sky cameras analyze the skysituationsinthevisiblespectralregion.Theresultsareusefulforinterpretingthesolar UV spectra measured by means of multifilter radiometers and spectrophotometers. Starting with a brief description of the state-of-art of the methodologies applied to cloud coverage evaluation, the new techniques developed to define, within the image framework, clouds in termsof area coverage and localization with respect to the Sunwill be reported.