Planning for the weather is an important factor for making your trip to Gero Onsen a success, especially when you’re splurging. 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.