The latest update for Pokémon GO implements a weather icon that can be tapped to highlight which Pokémon types are more likely to appear in the surrounding area and will be affected by current weather conditions. To see how this new feature works in Rain while a Mudkip is visible, check out the official video and screenshots embedded in the tweets below:
ゲーム内の「天気」マークから、今いる場所の天気状況を調べてみよう!天気によってトレーナーの皆さんが遭遇するポケモンの種類が変わります。 pic.twitter.com/lfSosMS29p
— Pokémon GO Japan (@PokemonGOAppJP) December 9, 2017

Source: @PokemonGoApp