Forecast

Full effect

During rain, strong sunlight, or hail, this Pokémon's type changes to Water, Fire, or Ice, respectively, and its form changes to match. This ability has no effect for any Pokémon other than Castform. If the weather ends or becomes anything that does not trigger this ability, or a Pokémon with Air Lock or Cloud Nine enters battle, this Pokémon's type and form revert to their default. If this ability is lost or disabled, this Pokémon cannot change its current type and form until it regains its ability.

In game description

The Pokémon transforms with the weather to change its type to Water, Fire, or Ice.

Using Forecast in Cobblemon

Abilities are passive and always active, so Forecast costs you nothing to run. If it is marked hidden on a species above, expect it to be harder to obtain: servers differ in whether hidden abilities come from breeding, an Ability Patch, or events.

See the Pokedex for which abilities each species can have, and IVs, EVs and breeding for how to actually pass one down.