Acid rain was unnaturally acidic rain.
Skaro, and especially Kaalann, had become so polluted that it constantly rained acid. (GAME: City of the Daleks)
Marran Alpha had constant acid rain. (AUDIO: The Dark Flame)
Thedion Four had constant acid rain. (TV: Mummy on the Orient Express)
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)