If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Sion: The hottest months are usually July and August with an average temp of 59°F, while the coldest months are January and February with an average of 29°F. The rainiest months in Sion are May, July, June, and August, with each month seeing an average of 8 inches of rainfall.