Acid rain means rain that has elevated acidic levels. There are many different forms of acid rain. It usually occurs when certain gases react in the atmosphere with a combination of oxygen and water.

Acid rain is a form of precipitation, like rain, that is abnormally acidic. That means that the ph. balance is off, and the rain has high levels of hydrogen ions. You can find more information here: http://en.wikipedia.org/wiki/Acid_rain...