In order to characterize the fluctuating nature of solar radiation in tropical climate, we classify daily distributions of the clearness index k(t) by estimating a finite mixture of Dirichlet distributions without assuming any parametric hypothesis on these daily distributions. The method is applied to solar radiation measurements performed in Guadeloupe (16 degrees 2N, 61W) where important fluctuations can be observed even within a period of a few minutes. The results exhibit four distinct classes of distributions corresponding to different types of days. The sequence of such classes can be of interest for future weather prediction. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.