ABSTRACT
Precipitation comprises all solid and liquid forms of water that are deposited on the Earth’s surface from the atmosphere. It includes rain, snow, hail, dew and sleet. All forms of precipitation are acid in so far as they have a pH of less than 7; in general, precipitation unaffected by human activity has a pH of 5.6. This naturally acidic state of precipitation is caused by the combination of water and carbon dioxide in the atmosphere to produce carbonic acid. However, the term acid precipitation, or acid rain, is usually applied to precipitation characterised by a pH of less than 5.1 (Elsworth 1984) and that contains sulphurous and nitrous acids. The latter are derived from various sources, among which fossil fuels are the most important.
