There is a continuous movement of water vapour into the upper atmosphere through evaporation from oceans, lakes, rivers and soil. On cooling, this water vapour condenses and falls as rain or snow. In the atmosphere it mixes' with variable amounts of naturally occurring carbon dioxide and sulphur and forms weak acids. Depending on the amount of carbon dioxide and sulphur present, the pH of the ensuing rainfall can be in the range 4.5-5.6. This artificial acidification of the cloud water results in an increase in the acidity of the precipitation, giving rise to regular occurrence of rainfall episodes with a pH less than 4.0.
Acids fall to the ground in particulate form.
This occurs downwind of the source of the emissions. The water bodies most susceptible to change due to acid precipitation are those whose catchments have shallow soil cover and poorly weathering bedrock, e.g. granite and quartzite. These soil types are characterised by the absence of carbonates that could neutralise acidity. The run-off water from such areas is less buffered than from areas such as limestone catchments, with an adequate level of carbonate. The principal areas with such characteristics are mostly associated with granite or quartzite bedrock.