The acidic precipitation that occurs when sulfur dioxides build up in clouds is called acid rain. The broader scientific term is acid deposition, which includes acidic rain, snow, fog, and dry particles.
Sulfur dioxide is released when fuels containing sulfur—especially coal and some oils—are burned. In the atmosphere, it can react with oxygen and water to form sulfurous and sulfuric acids. Nitrogen oxides from vehicles and power plants can create nitric acid as well, so modern acid precipitation is not caused by sulfur compounds alone.
Acid rain can lower the pH of lakes and streams, damage sensitive aquatic species, remove nutrients from soil, and weaken forests. It can also accelerate the weathering of limestone, marble, and some metals, which is why historic buildings and statues may suffer damage in polluted regions.
The problem became especially prominent in industrial areas during the 20th century. Emissions controls and cleaner fuels have reduced sulfur pollution in many countries, although acid deposition remains an environmental concern in some regions.