Garth's overly cautious, especially when it comes to his pokemon. Hoppip would love to play in snow because it is soft and sparkly and floats around like she does but Garth wouldn't want to risk her getting cold.
Glen and Riolu will go initiate snowball fights, then!
She's welcome to try! Igglybuff would try to explode but just burp instead because she'd not a geodude like she thinks she is.