Other facts from our historical weather data: Which months have the greatest rainfall in Sion? Most rainfall (rainy season) is seen in May, June, July and August. When is winter in Sion? The coldest season / winter is in the following months: January, February, November and December. Which is the warmest month in Sion? July has an average maximum temperature of 19°C (66°F) and is the warmest month of the year. Which is the coldest month in Sion? The coldest month is January with an average maximum temperature of -3°C (27°F). Which is the wettest month in Sion? June tops the wettest month list with 167mm (6.6 in) of rainfall. Which is the driest month in Sion? October is the driest month with 113mm (4.4 in) of precipitation. Which is the sunniest month in Sion? July is the sunniest month with an average of 270 hours of sunshine. No idea where to travel to this year? We have a tool that recommends destinations based on your ideal conditions. Find out where to go with our weather planner.