Derivation of cloud index from geostationary satellites and application to the production of solar irradiance and daylight illuminance data

We investigate in the present paper the relationship between satellite count, global irradiance and other solar and illumination resource components, bringing a particular attention to low solar elevation situations (below 20 degrees) which are very important in northern latitudes. Our investigation is based on data from two geostationary satellites, METEOSAT and GOES, backed by ground measurements in Switzerland and the northeastern USA. The study of different clear sky normalizations lead to the conclusion that a linear correlation between the global clearness index and the irradiance (like the heliosat method) would be inaccurate for low solar elevations, and therefore for high latitude regions. We developed a model that directly relates an elevation dependent clearness index to the could index. This methodology presents a definite advantage because it can be generalized to address the clearness index of other solar radiation components, besides global irradiance, such as direct irradiance, diffuse illuminance, etc. The correlations described in this paper were developed on the data from Geneva (in the frame of the EC program "Satellight") and evaluated on two other independent data sets (Albany, USA and Lausanne, Switzerland). Their precisions, on a hourly basis, are respectively 30%, 40% and 60% for the global, diffuse and beam components) (90, 55 and 95 W/m(2)). The use of independent data for the derivation and the validation of the models shows that those can be used in a wide range of locations, even if the applicability has to be assessed for very different climates.