Heavy Rain is another field-altering effect that you can get, this one by the ubiquitous Rain Dance move. As with Sunshine, it lasts five turns off its trigger move without a Damp Rock- and while it can be easier to find a pokemon that you don’t mind giving a Damp Rock to, it’s still a tough choice. Fortunately, Heavy Rain is also created by the Drizzle ability, which not only lets the Heavy Rain last until the weather is forcibly changed, but also is easier to get ahold of than Drought.
Like most weather effects, Heavy Rain has some base effects and then some individual effects that depend on a pokemon’s ability or moves. Also like the other major weather effects, Heavy Rain can completely alter the course of a battle, destroying most Fire pokemon tactics utterly and giving Grass types a few odd difficulties as well.
As its base effect, Heavy Rain cuts Fire attacks in half and increases Water attacks by half. This makes Heavy Rain probably the harshest crippling weather effect, because while Sunshine bothers Water pokemon, Sandstorm makes things generally difficult on anyone, and Hail mostly just injures almost everyone, Heavy Rain virtually shuts down most Fire pokemon- Water attacks are everywhere, and few Fire pokemon have a wide range of non-Fire attack options for dealing with the pokemon they usually come out against (Ice, Bug, and Grass).
Just to make things worse, Solarbeam, which is most Fire-types’ answer to an incoming Water pokemon, takes two turns to charge before firing in Heavy Rain, crippling that counter-option. Additionally, Heavy Rain improves the accuracy of both Thunder and Hurricane to 100, and grants them a 1-in-4 chance of piercing Detect or Protect. Interestingly, the fact that it boosts Thunder will sometimes make Heavy Rain very hazardous for Water pokemon, who otherwise have the most to gain from it. In addition to that, Heavy Rain cripples Synthesis, Morning Sun, and Moonlight, reducing them to 1/4 heals from their usual ½. Last of all, Heavy Rain makes Weather Ball a Water attack and doubles its power.
In terms of abilities, Heavy Rain enables Swift Swim and Hydration, and heals Dry Skin and Rain Dish pokemon by 1/16 of their HP every turn. Just to make things even better for Toxicroak, a Dry Skin pokemon in Heavy Rain also sucks up Water Attacks as though it had the Water Absorb ability. Finally, Castform becomes a Water type in the rain.
With all of this, Heavy Rain actually reduces very few opportunities- the light-dependent healing moves aren’t very popular even on grass pokemon, and few people heavily implement Fire-types that don’t have a wide range of attack options anyways. More importantly, because Thunder becomes a full-accuracy move (nevermind breaking annoying Detect and Protect walls), Heavy Rain improves your ability to use both Water and Electric pokemon- and especially those Water pokemon that are capable of learning Thunder (particularly Slowbro/Slowking and Starmie, just for the first couple of examples to come to mind). Toxicroak, as mentioned, goes from being decent to being fairly good and competing well with other sweepers thanks to the improved durability. And many not-so-remarkable pokemon gain the ability to develop quickly into powerhouses (I’m looking at you, Swift Swim Dragon Dance Kingdra). Heavy Rain is probably the easiest weather condition to take advantage of, and the variety of team members that go well with it is immense.
