Missed Clouds in the NSRDB at Low Sun Elevation Angles

The NSRDB PSM dataset has been found to periodically falsely identify cloudy periods as clear-sky at low solar elevations. An example case confirmed that this is caused by false negative identification of clouds by the cloud detection model. This modeling artifact results in a positive bias of the estimated solar irradiance, particularly for tracking systems. Rough estimations showed that the bias may be up to 1%, which can have significant consequences for PV developers.