Planning for the weather is a good way to make your trip to Sarnen relaxing and enjoyable, especially when you're splurging. The hottest months are usually July and August, with an average temperature of 13°C, while the coldest months are January and February, with an average of -3°C. The rainiest months in Sarnen are June, July, August and May, with each month seeing an average of 222 mm of rainfall.