Drought summons sunlight in battle as soon as a Pokémon with Drought enters the battle. Its effects almost are the same as Sunny Day's; the only difference is that instead of lasting for five turns, it lasts for the whole battle until a different weather condition replaces it. If a Pokémon with Drought is sent out in the same turn as another Pokémon whose Ability creates a different weather condition, the slower Pokémon's Ability will override the faster Pokémon's Ability.
In Generation VI, the sunlight will last only for five turns instead. This can be extended to eight turns if the Pokémon is holding a Heat Rock.