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, mist, dry dust, hail, or sleet. This can add to the acidity of the soil, and affect the element balance of lakes and streams.The term "acid rain" is sometimes used more usually to include all forms of acid deposition - both wet deposition, where acidic gases and particles are removed by rain or other precipitation, and dry deposition removal of gases and particles to the Earth's surface in the absence of precipitation. Acid rain is defined as any type of precipitation with a pH that is abnormally low. Dissolved carbon dioxide dissociates to form weak carbonic acid giving a pH of approximately 5.6 at typical atmospheric concentrations of CO2. Therefore a pH of less than 5.6 has every so often been used as a definition of acid rain. However, natural sources of acidity mean that in remote areas, rain has a pH which is between 4.5 and 5.6 with an average value of 5.0 and so rain with a pH of less than 5 is a more appropriate definition. The US EPA says, "Acid rain is a serious environmental problem that affects large parts of the US and Canada" Acid rain accelerates weathering in carbonate rocks and accelerates building weathering. It also contributes to acidification of rivers, streams, and forest damage at high elevations. When the acid builds up in rivers and streams it can kill fish.