- precipitation, as rain, snow, or sleet, containing relatively high concentrations of acid-forming chemicals, as the pollutants from coal smoke, chemical manufacturing, and smelting, that have been released into the atmosphere and combined with water vapor: harmful to the environment.
- rain that contains a high concentration of pollutants, chiefly sulphur dioxide and nitrogen oxide, released into the atmosphere by the burning of fossil fuels such as coal or oil
