The term acid rain or more accurately acid precipitation is commonly used to mean the deposition of acidic components in rain, snow, dew, or dry particles. Acid rain occurs when sulfur 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, mist, dry dust, hail, or sleet. This increases the acidity of the soil, and affects the chemical balance of lakes and streams. (wikipedia)