This article calculates day-head forecast variability as function of historical clearness index based on intraday irradiance variability. In this paper we address the use of satellite data as a proxy for ground data to determine the clearness index of a day, and hourly clearness index variability. The article also demonstrates the application of the intraday variability to predict day-ahead hourly forecast variability range as a function of the predicted day’s clearness index.