Continuous measurements of sky radiation have been performed in Geneva for more than two years. We present here some results, and we show that the radiative earth-sky deficit is correlated to the clearness of the day as evaluated from the amount of solar radiation reaching the ground. The corresponding model allows to determine the sky temperature within a 2-3°C, with respect to the ambient temperature, diffuse and global solar radiation. These results can be of help when computing energy balance involving sky radiation, especially in buildings and solar energy fields.