Pokemon Go is making some huge changes and it's all tied to what the weather's like outside.
Earlier today, Pokemon Go announced that they were adding a dynamic weather system to the game, one that would change what type of Pokemon would spawn based on the weather. Pokemon that spawn because of the weather will have a weather effect surrounding them, similar to how the game marks Pokemon that spawn because of incense. Certain types of Pokemon moves will also become more powerful in certain types of weather, adding to their effectiveness in gyms and raids.
Pokemon Go is introducing six different types of weather to the game, each of which impacts different Pokemon. The six different weather patterns (and which type of Pokemon they affect) are as follows:
Windy Weather: Dragon-Type, Psychic-Type, Flying-Type
Foggy Weather: Dark-Type, Ghost-Type
Clear/Sunny Weather: Grass-Type, Fire-Type, Ground-Type
Snowy Weather: Ice-Type, Steel-Type
Rainy Weather: Water-Type, Bug-Type, Electric-Type
Cloudy Weather: ???
We haven't seen which Pokemon will get an advantage in cloudy weather, but Normal-Types, Fighting-Types, Poison-Types, Rock-Types, and Fairy-Types all aren't represented in the other weather patterns.
When players catch one of the "boosted" types of Pokemon during these weather patterns, they'll also receive 25 extra Stardust as a reward.
However, Niantic confirmed to the Verge that Pokemon wouldn't be penalized in certain weather patterns, so there wouldn't be a penalty for using Fire-Type Pokemon during rainy weather. “We prefer to give bonuses rather than penalties where we can,” a Niantic official told the Verge.0comments
That same official also confirmed that certain types of Pokemon won't ONLY appear during certain weather conditions. So while it's more likely that you'll see a Lotad during rainy weather, it's still possible to find and catch one on a sunny day.
Don't expect to see the weather effects immediately. The Verge noted that, while Gen 3 Pokemon will be added to the game sometime this week, the weather effects will be added in a second update. The weather effects will be added to the game "later this month."