Acid rain is rain, snow or fog that is polluted by acid in the atmosphere and damages the environment. Rain is naturally acidic because carbon dioxide, found normally in the earth's atmosphere, reacts with water to form carbonic acid. The acid in acid rain comes from two kinds of air pollutants-- sulphur dioxide (SO2) and nitrogen oxides (NOx). These are emitted primarily from utility and smelter "smokestacks" and automo