To tie in with the release of the 3rd generation Hoenn Pokemon, Pokemon GO introduced the newest concept of weather. Weather is an effect that will now always be in play while playing Pokemon GO, tieing in with the real world weather of your given region. (Weather will update every hour or so during gameplay, not changing in real time.) As the weather changes within the game, certain Pokemon will be affected depending on the type of weather that is begin seen.
Pokemon that are affected by a certain type of weather will appear more often and have much better ivs than normal, while Raid Bosses will have a much higher combat power Level if their given typing is affected by the weather you are experiencing. Certain Type of Pokemon Moves will also see a 20% increase if used in the corresponding weather, such as Fire Type Attacks doing more damage during Sunny Weather. Players will also receive extra stardust for capturing a Pokemon that is being affected by the weather around them.
Sunny weather: Affects grass types, ground types & fire types
Clear weather: Affects grass types, ground types & fire types
Partly Cloudy weather: Affects normal types & rock types
Cloudy weather: Affects fairy types, fighting types & poison types
Rainy weather: Affects water types, electric types & bug types
Snowy weather: Affects ice types & Steel Types
Windy weather: Affects dragon types, Flying Types & psychic types
Foggy weather: Affects ghost types & dark types