This paper studies dynamic processes of fast-alternating solar radiation which are assessed by alternation of clouds. Most attention is devoted to Clouds of type Cumulus Humilis, identified through visual recognition and/or a specialty constructed automatic sensor. One second sampling period was used. Recorded data series were analyzed with regard to duration of illuminated 'windows' between shadows, their stochastic intervals, fronts and the magnitude of increments of solar irradiance. (C) 2009 Elsevier Ltd. All rights reserved.