Even the largest photovoltaic (PV) power plants are widely affected by overpassing cloud shadows which have several harmful effects on the operation of PV systems. Irradiance transitions caused by edges of cloud shadows can be very steep and large and might lead to situations where the grid inverter is not able to follow the global maximum power point. Further, partial shading of PV systems causes mismatch losses and fast fluctuations of the power fed to the electric grid can cause power balance and quality problems. In this paper, a method to determine apparent shadow edge velocity from measured data of three irradiance sensors is presented. A total of around 43,000 irradiance transitions were first identified in 15 months of data measured with one of the sensors around midsummer in 2011-2014. Out of those about 27,000 transitions were identified by all the three irradiance sensors and their apparent shadow edge velocity, length, etc. were analysed. The apparent shadow edge speed varies greatly with an average value of around 9 m/s. The lengths of irradiance transitions caused by edges of moving clouds are typically around 100 m, which is large enough to affect the operation of PV power plants of all sizes. (C) 2016 Elsevier Ltd. All rights reserved.