In the United States, acid rain is most prevalent in the Northeast. Southeast Canada and much of Europe, including Norway and Sweden, experience acid rain. Acid rain is a major problem in developing countries.
The large number of cities, high population density and concentration of power and industrial plants in the Northeast cause patterns of high acidity in that area. The wind direction from the Midwest to the Northeast also brings storms and pollutants. In the middle of the United States, precipitation is most acidic downwind of the concentration of power plants in the Ohio Valley.
In general, rain is considered acidic when its pH value is below 5.3. Most of the rainwater that falls in the eastern United States has a pH between 4.0 and 5.0. Acid rain can precipitate in the form of snow, rain, fog, smoke or dust. Acid rain occurs when nitrogen oxides and sulfur dioxide react with water in the air. Water droplets in clouds absorb the particulates and return them as snow, sleet or rain. Acid rain increases the acidity of soil and disrupts the chemical balance of water bodies. Most particles with a diameter of 10 microns or fewer remain in the atmosphere, causing health problems.