Acid rain was unnaturally acidic rain.
Skaro, and especially Kaalann, had become so polluted that it constantly rained acid. (GAME: City of the Daleks)
In the early 21st century, acid rain was more common due to high pollution levels. (PROSE: Cat's Cradle: Warhead) By the mid-21st century, acid rain had become corrosive. (COMIC: Strange Loops)
Marran Alpha had constant acid rain. (AUDIO: The Dark Flame)
Thedion Four had constant acid rain. (TV: Mummy on the Orient Express)
In 3764, a Jinglatheen death-squad who commandeered the Clixian weather station caused a bout of acid rain to occur on Clix. Though it had no effect on humans, Raxacoricovarlonpatorians or Clixians, it resulted in numerous Raxacoricofallapatorians being burned to death. The Jinglatheen had intended to exploit this tragedy to gain control of the Raxas Alliance, only to be exposed by Slist Fayflut Marteveerthon Slitheen. (COMIC: Doctormania)
In 5345, the harvest rangers used satellites to produce acid rain to melt down Androzani trees for fuel. (TV: The Doctor, the Widow and the Wardrobe)
Acid rain fell on Red Rocket Rising after it was struck by an asteroid. (AUDIO: Blood of the Daleks)