acid rain

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Acid precipitation falling as rain.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Rain which is unusually acidic (pH of less than the natural range of 5 to 6); caused mainly by atmospheric pollution with sulphur dioxide and nitrogen compounds.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun rain containing acids that form in the atmosphere when industrial gas emissions (especially sulfur dioxide and nitrogen oxides) combine with water