Weather is probably a major factor when you plan your trip to Bandai Atami Onsen, and keep in mind that cheaper options are usually available when the weather isn't as good. 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.