English: This grey glowing "cloud" over the city of Berlin is due to light scattered off of suspended aerosols below the planetary boundary layer (PBL). The sky above the PBL appears dark because the air is cleaner, giving the upward propagating light less opportunity to scatter towards the viewer. Such a clear view of the PBL is only possible when the viewer is at approximately the same altitude as the layer. The bright bluish spot on the photo is Olympiastadion Berlin