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From Bulbapedia, the community-driven Pokémon encyclopedia.
Acid rain is a glitch in Pokémon Platinum, HeartGold and SoulSilver that results in the simultaneous occurrence of all weather conditions at once, thus damaging all Pokémon on the field. Even though it was discovered before the English release of Platinum, it is present in all versions of the mentioned games.
If any weather effect (including Trick Room, Gravity, and Uproar, but excluding rain) is in effect, and a player of Platinum, HeartGold, or SoulSilver uses Pursuit on a Pokémon that is switching out on that turn and causes it to faint, this glitch will activate. Upon sending out the next Pokémon, the weather will have changed, and all four weather conditions will be occurring at once until the end of the battle.
This will occur if the user of Pursuit is the host, even if the host game is Diamond or Pearl.
If Trick Room, Gravity, or Uproar is in effect when the glitch is activated, they will activate the glitch, along with any other effects with priority below it. The priority is:
Fog (hurts after animation)
Hail (hurts after animation)
Sunshine (hurts after animation)
Sandstorm (hurts after animation)
Rain (hurts after animation; glitch cannot be activated using this weather)
Other moves with multiple-turn effects, such as Mud Sport, Water Sport, Tailwind, and Perish Song, will not be affected and work normally.
Even if the Pokémon is normally immune to a weather condition due to its type (Ice-type Pokémon with hail, and Rock-, Ground-, and Steel-type Pokémon with a sandstorm), they will accumulate damage four times each turn, with the cause of the damage identified as their Ability, rather than the weather. In this way, a Pikachu in battle can be hurt by its Static, a Weezing by its Levitate, or an Empoleon by its Torrent.
However, if the Pokémon is immune to both Hail and Sandstorm, like Swinub, that Pokémon will be immune to all of the damaging effects of this glitch.
Conversely, a Pokémon with an Ability that heals it in certain weather conditions, such as Ice Body or Rain Dish, will not take damage, but instead be healed four times every turn until they are fully healed, and then, the weather will hurt it. Dry Skin, which heals in some weather and hurts in other weather will work as it normally does, healing with one condition and hurting with the other.
Both Thunder and Blizzard have 100% accuracy, due to simultaneous rain and hail. SolarBeam will require no charging due to the sun, but its damage will also become decreased by 50% due to the other three weather conditions.
If a Castform or Cherrim is brought into battle, their Abilities will cause them to constantly transform, thus locking up the battle until the game is turned off. Castform will only transform between its Sun and Rain forms due to those particular weather conditions being the first two to occur (and thus, Castform recognizes these as being the only two weather conditions on the field).