Pokémon Black Version and Pokémon White Version - Weather
The weather moves can be used at any time, even during weather. A weather move fails if the weather condition associated with that move is already in effect. For example, Rain Dance fails while the weather is rainy.
The effects of Hail, Rain Dance, Sandstorm, and Sunny Day, are weather effects. (Fog appears not to exist.) The effects of Gravity and Trick Room are not weather effects.
There can be only one weather condition at a time. If another weather condition comes in effect, the previous condition is canceled.
In some battles, a particular weather condition already comes in effect before the first turn of the battle. This condition lasts indefinitely. If a Pokémon has an Ability that makes the weather change to a different condition, it then takes effect.