Accurate surface radiation fluxes are required to quantify atmosphere-surface energy exchanges. In climate research, detailed investigations of the influence of diverse atmospheric conditions on radiative fluxes ask for a well-defined distinction between clear-sky and cloudy-sky situations. Effects of clouds, the so called cloud forcing, can only be determined by explicitly separating clear-sky from all-sky radiation measurements. A Clear-Sky Index (CSI) to separate clear-sky from cloudy-sky situations has therefore been introduced, using accurate atmospheric longwave radiation in conjunction with air temperature and humidity measurements at the station. This clear-sky index, which has the important advantage to be applicable 24 hours a day, was tested and first used on measurements of the Alpine Surface Radiation Budget (ASRB) network, and was of prime importance for the determination of the altitude dependence of cloud forcing and greenhouse effect over the Alps. ¿ 2000 American Geophysical Union