Weather Boost

The weather in Pokémon GO follows the real weather where you are, and each kind of weather boosts two or three types. A Pokémon is boosted if either of its types is. For example, Eternatus is a Veneno and Dragón type Pokémon, so it is boosted in both Cielo cubierto and Viento weather.

Boosted types

In battle

Moves of a boosted type deal 20% more damage in raids, gyms and Max Battles. The move's type is what counts, not the Pokémon's: a Water-type Pokémon's Rayo Hielo is boosted in Nieve weather. There is no damage boost in Trainer Battles, the GO Battle League or Team GO Rocket battles.

Catching

• Wild Pokémon of the boosted types appear more often.
• Weather-boosted Pokémon are 5 levels higher: wild Pokémon reach level 35 rather than 30, Pokémon caught after a raid are level 25 rather than 20, and Shadow Pokémon rescued from Team GO Rocket are level 13 rather than 8. Each Pokémon's page lists its max CP with weather boost.
• Each IV of a weather-boosted wild Pokémon is at least 4. Raid catches keep their usual minimum of 10.
• Catching a weather-boosted Pokémon gives 25% more Stardust.

Evolution

Sliggoo evolves only in rain, or near a PokéStop with a Rainy Lure Module. The evolution requirements guide lists the other Pokémon that evolve near a Lure Module.