This diagram shows the water cycle and how contamination can lead to acid rain. Acidic gases are released into the atmosphere through car emissions and industries. Sulphur dioxide and nitrogen oxide are the two main gases that are released into the atmosphere from factories and nitrogen oxide is released from the car emissions. These gases are carried by the wind and they dissolve in rain water to form acid precipitation. Acid precipitation can occur in many forms and returns to earth in the form of rain, snow, sleet, and fog.