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All effects of fog, hail, rain, sandstorm, and sun are temporarily disabled as long as the wielder is in play. The weather condition will continue to prevail until it fades or is replaced, but the battle will act as if there is no weather condition in play. However, the game will report the messages about what weather is active. The weather-related formes of Cherrim and Castform will revert to normal when the wielder is on the field. When a Pokemon with Cloud Nine enters battle, an activation notification of "[Pokemon] has Cloud Nine!" will be given.
In a metagame dominated by weather, where sandstorm and rain are never too far away, Cloud Nine can be an incredibly useful tool, capable of stopping teams in their tracks. Although its distribution leaves a lot to be desired, Altaria and Golduck are by no means poor when it comes to stopping rain teams or sandstorm teams. There may be more effective checks for certain weather conditions, but Cloud Nine is more than sufficient as a temporary stop to any weather abuser's sweep.
Found in Dream World