Weather

Weather is an in-game phenomenon that mimics a Trainer's real-world weather experience. The current weather is reflected in the Map View and capture phase. Weather conditions make Pokémon of certain types stronger and cause their attacks to be more effective. Pokémon will appear more frequently in wild and award more Stardust when captured; depending on what the weather condition is.

Types of weather
There are eight types of weather conditions that occur in-game.

Clear
Clear weather, also referred as Sunny.

Boosted types:  Fire Grass Ground Full weather images: 

Partly Cloudy
Boosted types:  Normal Rock Full weather images: 

Overcast
Boosted types:  Fairy Fighting Poison Full weather images: 

Windy
Boosted types:  Dragon Flying Psychic Full weather images: 

Rain
Boosted types:  Bug Electric Water Full weather images: 

Fog
Boosted types:  Dark Ghost Full weather images: 

Snow
Boosted types:  Ice Steel Full weather images: 

Extreme
If the weather outside is too dangerous, the game will issue a severe weather alert to notify the Trainer that they should seek shelter.

Full weather image: 

Weather Icons


Trivia

 * This feature was announced along with Pokémon from Hoenn region on December, 2017, and released two days later.
 * Several weather conditions like Rain, Wind or Snow causes respective visual effects in Pokémon encounter screen.
 * Pokémon boosted by weather appear with additional spinning, pale-blue rings beneath them in the Map View when they spawn.
 * The same Pokémon spawn's stats will apparently be updated as the weather changes if you haven't caught it yet, but when you catch them, they would revert to their originally spawned stats. Stardust gained is also not boosted.
 * Before the release of the Sinnoh Pokémon, Weather had a higher effect on the rate at which Pokémon appeared.