acid rain

n. polluted rain that contains sulfur dioxide and nitrogen oxides which is caused by air pollution

The term "acid rain" is commonly used to mean the deposition of acidic components in rain, snow, fog, dew, or dry particles. The more accurate term is "acid precipitation." Distilled water, which contains no carbon dioxide, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are alkaline (or basic). "Clean" or unpolluted rain has a slightly acidic pH of 5.6, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid. The extra acidity in rain comes from the reaction of air pollutants, primarily sulfur oxides and nitrogen oxides, with water in the air to form strong acids (like sulfuric and nitric acid). The main sources of these pollutants are vehicles and industrial and power-generating plants.
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Noun
1. rain containing acids that form in the atmosphere when industrial gas emissions (especially sulfur dioxide and nitrogen oxides) combine with water
(synonym) acid precipitation
(hypernym) air pollution

is rain that is more acidic than normal. Caused by air pollution, acid rain’s spread and damage involves weather, chemistry, soil, and the life cycles of plants and animals on the land and from acid rain in the water. Air pollution from the burning of fossil fuels is the major cause of acid rain. Smoke and fumes from burning fossil fuels rise into the atmosphere and combine with the moisture in the air to form acid rain. The wet forms can fall as rain, snow, or fog. The dry forms are acidic gases or particulates.

(See: acid deposition.)