Actually, comrade, Acid Rain(which is radioactive rain) is highly concentrated, and can clear up an entire area(cloud sized area, as the cloud is the concentrate mechanism) at a time. Depending on how heavily it rains, and how wide-spread it determines how fatal the rain can be to people. Acid ran can do anything from simply strip plant life, to kill wildlife.(Over the course of the storm.) Acid Rain would essentially cleanse the sky, but douse the land it rained on in Radiation. So, after a long enough time-frame, the sky would hold little to no radiation that would actually press an issue for ANY form of life.