Pokémon Diamond Version, Pearl Version, and Platinum 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, as well as fog, are weather effects. The effects of Gravity and Trick Room are not weather effects.
There can be only one weather condition at a time (but see below). 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 unless noted otherwise. If a Pokémon has an ability that makes the weather change to a different condition, it then takes effect.
If two or more Pokémon with an ability that can change the weather enter the battle at the same time, their effects are resolved in turn order; that is, the ability of the Pokémon that would strike last in turn order will ultimately take place.
Because of a bug in Pokémon Platinum Version (and probably also HeartGold and SoulSilver), if a Pokémon causes a target to faint with Pursuit as it’s about to switch while a weather condition or similar effect is in effect (as listed below), the game erroneously activates other effects that "come after" it in the following order: Trick Room, fog, Gravity, Uproar (only the effect that provides that "no Pokémon can fall asleep and each sleeping Pokémon in battle wakes up at the beginning of its attack segment"), Hail, Rain Dance, Sandstorm, and Sunny Day. This phenomenon is sometimes known as "acid rain" or "acid weather." For example, if Sunny Day is currently in effect, the effects of Sandstorm and Rain Dance also come in effect simultaneously.