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.

WHAT IS RAIN?

Most rain falls over the oceans, but about one fifth falls on land of which about 40% evaporates, the remainder flowing back to the seas through streams and rivers.

WHAT IS ACID RAIN?

"Acid Rain", or more correctly acid precipitation, is the term used to describe polluted rainfall. When the water vapour from the oceans and the land enters the atmosphere it is neutral in reaction i.e., non-acid, pH 7, and is almost pure H20. In the atmosphere it mixes with variable amounts of naturally occurring carbon dioxide (CO2) and sulphur (S) and forms weak acids. Depending on the amount of CO2 and S present, the pH of the ensuing rainfall can be in the range 4.5-5.6. In the industrialised nations of Europe and North America heavy combustion of fossil fuels (oil and coal) results in the emission of sulphur dioxide (SO2) and nitrogen oxides (NOx) into the atmosphere. Transformation of these gases into sulphuric acid (H2SO4) and nitric acid (HNO3) leads to further acidification of the water vapour. This artificial acidification of the cloud water (Fig. 1) 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.