This paper investigates the relationship between two parameters characterizing a given location on a given day: the daily clear sky index KT* and the intraday variability given by the standard deviation of the changes in the hourly clear sky index sigma(Delta kt(Delta t)*). Empirical evidence assembled from twenty climatically distinct locations led us to derive a simple model to infer intraday variability from the day's clear sky index. Although the model shows little dependence on location, we did observe a systematic difference traceable to a location's prevailing cloud formation regime. Therefore, we also propose two alternative models for sites where cloud formations is influenced by local orography and sites where cloud formation is traceable to weather events only. Finally, we present a possible application of the models to enhance the informative content of day(s) ahead NWP forecasts. (C) 2015 Elsevier Ltd. All rights reserved.