Forecast

 is a Pokémon ability introduced in the third generation. It is the signature ability of Castform.

Description
Forecast changes Castform's form depending on the weather. The form changes are It will revert to its normal form if the weather is normal, sandstorm, fog, shadowy aura or mysterious air current. It will also revert if a Pokémon  with Cloud Nine or Air Lock is brought out.
 * Sunny Form (Fire type) - in harsh sunlight
 * Snowy Form (Ice type) - in hail
 * Rainy Form (Water type) - in rain

The form change is after Castform is brought out so entry hazards treat it like a Normal type.

Forecast cannot be copied by Role Play, Trace or Power of Alchemy and another Pokémon cannot be changed to Forecast by Entrainment. If Castform with Forecast is hit by Gastro Acid, Castform will turn back to its normal form. When Castform is transformed into, that Pokémon stays in the same form regardless of the weather.