WHEN SOMETHING IS perfect, people say that it is "right as rain." For instance, when someone is not feeling well, he or she might be told to go to bed early, to get plenty of sleep, and to expect to feel "right as rain" in the morning. It is a popular saying. People also imagine that rain is pure. Because it falls from high above, rain is assumed to be uncontaminated by the dirt and debris of the earth. In fact, when rain falls, it is thought to cleanse the landscape below.

At least that is what people have always thought. However, since the 1980s, scientists have been saying that the rain is not "right" or clean at all.

Dying forests in Europe and dying lakes across the world were commonplace stories on the news in the 1980s. At first...