Weather is probably a major factor when you plan your trip to Gero 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 66°F, while the coldest months are January and February with an average of 27°F. The rainiest months in Gero Onsen are July, September, August, and June, with each month seeing an average of 13 inches of rainfall.