Acid Rain

Acid Rain does one job: while it lasts, fruit has a higher chance of picking up the Acidic mutation. The wiki gives Acidic a x15 multiplier and names Acid Rain as its condition, so this is the weather to wait for if you want that mutation on your harvest.

Nothing else is attached to it. Unlike Rain, which adds +50% grow speed along with its Wet chance, the Acid Rain entry mentions no growth bonus and no rewards, only the mutation roll. Its x15 matches the multiplier of Boil from Boil Rain, so the two sit at the same level.