Acid rain occurs when sulphur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain, snow, or sleet. This can increase the acidity of the soil, and affect the chemical balance of lakes and streams.

Acid rain is defined as any type of precipitation with a pH that is unusually low. Acid rain accelerates weathering in carbonate rocks and accelerates building weathering. It also contributes to acidic rivers, streams, and damage to trees at high elevation. Efforts to combat this phenomenon are ongoing.

Acid rain was first reported in Manchester, England, which was an important city during the Industrial Revolution. In 1852, Robert Angus Smith found the relationship between acid rain and atmospheric pollution. He used the term “acid rain” for the first time in 1872.

Though acid rain was discovered in 1852, it wasn't until the late 1960s that scientists began widely observing and studying the phenomenon. Canadian Harold Harvey was among the first to research a "dead" lake. Public awareness of acid rain in the U.S increased in the 1990s after the New York Times promulgated reports from the Hubbard Brook Experimental Forest in New Hampshire of the myriad deleterious environmental effects demonstrated to result from it.

Evidence for an increase in the levels of acid rain comes from analysing layers of glacial ice. These show a sudden decrease in pH from the start of the industrial revolution of 6 to 4.5 or 4. Other information has been gathered from studying organisms known as diatoms, which inhabit ponds. Over the years these die and are deposited in layers of sediment on the bottoms of the ponds. Diatoms thrive in certain pHs, so the numbers of diatoms found in layers of increasing depth give an indication of the change in pH over the years.

Since the industrial revolution, emissions of sulphur and nitrogen oxides to the atmosphere have increased. Industrial and energy-generating facilities that burn fossil fuels, primarily coal, are the principal sources of increased sulphur oxides. Occasional pH readings of well below 2.4 (the acidity of vinegar) have been reported in industrialized areas. These sources, plus the transportation sector, are the major originators of increased nitrogen oxides.

Industrial acid rain is a substantial problem in China, Eastern Europe, Russia and areas down-wind from them. Acid rain from power plants in the midwest United States has also harmed the forests of upstate New York and New England. These areas all burn sulphur-containing coal to generate heat and electricity.

The principal cause of acid rain is sulphur and nitrogen compounds from human sources, such as electricity generation and motor vehicles. The gases can be carried hundreds of miles in the atmosphere before they are converted to acids and deposited.