If you're looking forward to lots of outings with your partner, check out these details about year-round temperatures in Bandai Atami Onsen: The hottest months are usually August and July with an average temp of 70°F, while the coldest months are January and February with an average of 33°F. The rainiest months in Bandai Atami Onsen are July, September, August, and October, with each month seeing an average of 8 inches of rainfall.