If you’re looking forward to lots of outings with your partner, check out these details about year-round temperatures in Shiozawa Onsen: The hottest months are usually August and July with an average temp of 66°F, while the coldest months are January and February with an average of 27°F. The rainiest months in Shiozawa Onsen are July, September, August, and June, with each month seeing an average of 13 inches of rainfall.