Speaking generally, cloud cover over the primary target meant looking at a list of alternatives. The Germans usually set out drums that released smoke near targets, and which ones were lit depended on the prevailing winds. The bomber stream could not move in an uncoordinated way either. Bombers that were badly damaged and losing altitude, would jettison their bombs at some obvious landmark. In one case, this was where two rivers crossed each other. Other times, such landmarks were not so clear from altitude.
https://www.8thafhs.org/old/new/get_...mission_id=863
https://www.criticalpast.com/video/6...ld_aerial-view