Drizzle summons rain in battle as soon as a Pokémon with Drizzle enters the battle. Its effects are the same as the move Rain Dance, but instead of lasting for a limited amount of turns, it lasts for the whole battle (unless it is replaced by another type of weather). If a Pokémon with Drizzle is sent out in the same turn as another Pokémon whose Ability affects the weather, the slower Pokémon's Ability will override the faster Pokémon's Ability.
The rain now lasts only for five turns. This can be extended to eight turns if the Pokémon is holding a Damp Rock.
In Super Mystery Dungeon, Drizzle changes the weather to rain upon entering a floor. Rain lasts a certain number of turns (in multiples of 66) before clearing. After the weather has cleared, Drizzle will again activate after waiting a certain number of turns (in multiples of 66).