When rain water in clouds get mixed with sulphur dioxide and oxygen present in atmosphere, this mixture (i.e., sulphuric acid) is called acid rain.
It is caused mainly due to burning of coal which contains sulphur as impurity, and emits sulphur dioxide on burning. This sulphur dioxide in the atmosphere reacts with oxygen to form sulphur trioxide which in turn, reacts with the water in the clouds to form sulphuric acid.
2SO2(g) + O2(g) 2SO3(g)
Sulphur dioxide Oxygen Sulphur trioxide
SO3(g) + H2O(l) H2SO4(l)
Sulphur trioxide Rain water Sulphuric Acid (Acid Rain)