Falling rain (or snow) which has become acidic
(pH less than 5.6) as a result of its
combination with gaseous pollutants, such as sulphur dioxide (SO2)
and nitrogen oxides (NOx).
Acid rain may cause acidification of surface waters, soils and ecosystems.
Source: GreenFacts
More:
Victims of acid rain - dead and dying Red Spruce in Maine
