In photovoltaic (PV) power plant construction, solar resources need to be evaluated using some criteria. Besides considering the classical total amount of solar resources at the considered location, the lengths and distributions of equivalent clear-sky periods have also been taken into account in recent years to consider the stability of both the PV system output power and other connected grids. We present a method of identifying the lengths and distributions of equivalent clear-sky periods using time series of direct normal irradiance (DNI) measurements based on generalized atmospheric turbidity. This method can be employed not only to calculate the lengths and distributions of historical equivalent clear-sky periods, but also for real-time equivalent clear-sky condition detection. In this report, we demonstrate the validity our algorithm using data from the Solar Radiation Research Laboratory (39.74 degrees N, 105.18 degrees W) and compare our results with those obtained using the Reno and Hansen algorithm. (C) 2019 Elsevier Ltd. All rights reserved.