Sorry to rain on your parade...
Pokémon GO has been suffering from a bug for over 5 months now, but Niantic has finally fixed up the issue. It’s not a major one, but it’s certainly an element that helps enhance immersion in the game.
5+ months back, Niantic announced that they were disabling some weather-related visual effects in Pokémon GO due to a bug. This only impacted the game visually, but it was still a sticking point for fans. We’re not quite sure why it took over 5 months to fix the issue, but Niantic has revealed that weather-related visual effects have indeed returned.
If you’re still not seeing the correct weather representation in-game, all you have to do is restart the app. That should set everything straight, and in-game weather visuals will appear.