Heavy Rain

The Heavy Rain weather condition was introduced in the second generation of Pokémon games (GSC). It can be brought about by use of the move Rain Dance (TM18) or by the ability Drizzle.

Heavy Rain will not damage any type of Pokémon inbetween turns in battle (unless an ability would cause HP loss).

A Pokémon holding the item Damp Rock will prolong the duration of Heavy Rain for three more turns than normal (from five turns to eight turns).

Heavy Rain's affect on Abilities

Ability	Weather Effect
Drizzle	This ability will summon rain which will last the entirety of the battle, unless another weather condition is introduced to the battlefield or it's effect is nullified by Cloud Nine or the Air Lock abilities.
Dry Skin	A Pokémon with this ability will recover 12.5% of it's maximum HP at the end of every turn. It will also recieve a 25% increase in maximum HP if attacked with moves.
Hydration	This ability will remove any status condition when in battle as long as it is raining. This includes removal of Sleep after use of Rest.
Rain Dish	6.25% (1/16) of this Pokémon's maximum HP is restored at the end of every turn when it is raining.
Swift Swim	The Speed stat of Pokémon with this ability is doubled.

Heavy Rain's affect on Moves

Move	Weather Effect
Rain Dance	This move will summon (or prolong) Heavy Rain for 5 turns.
Morning Sun	25% (1/4) of the user's HP is restored, when Morning Sun used.
Moonlight	25% (1/4) of the user's HP is restored, when Moonlight used.
Synthesis	25% (1/4) of the user's HP is restored, when Synthesis used.
Solarbeam	Solarbeam's base power drops 50% (from 120 to 60).
Thunder	Thunder's accuracy is boosted to 100% (for a gain of 30% it's initial accuracy of 70%) . It has the potential to break through Detect and Protect barriers at a rate of about 1 in 3 (30% of the time).
Weatherball	Weatherball becomes type. It's base power doubles from 50 to 100. Castform will recieve STAB from Weatherball in Heavy Rain.
Generalized Effects
	Fire type moves get a reduction of 50% (or a decline in strength of .5 times) their initial value.
	Water type moves get a boost of 150% (or an increase in strength of 1.5 times) their initial value. This ability does stack on initial STAB (Same Type Attack Bonus).

Heavy Rain's affect on Castform

The ability Forecast will affect Castform based on what weather condition is active, if any. In this case...

#351 - Castform (Rain Forme)

Typing Becomes: