In-game description: Summons rain in battle.

Technical explanation and other notes
If a Pokemon with this Ability is sent into battle, it will also summon Rain to fall on the battlefield. This weather lasts until the battle is over or is replaced by another weather modifying ability. When 2 Pokemon with conflicting weather Abilities enter the field of play at the same time, the slowest one's weather effect lasts.

Pokémon with this ability