Note that naturally occurring CO₂ in the atmosphere dissolves in rainwater and makes it slightly acidic (this will be the background, "natural" acidity of rain). Any pH much lower than this (5.6) is considered "acid rain". The acid in acid rain may be sulfuric or nitric acid or a combination of both.

The extreme cases of acid fog occur when the fog droplets are evaporating, or the fog is "burning off". Evaporation of the droplets reduces their size, but the acid gets left behind in the smaller droplet. This causes the acid to become more concentrated, so the lowest pH (most highly concentrated acid) occurs just as the fog "lifts".