If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Iwamuro Onsen: The hottest months are usually August and July with an average temp of 73°F, while the coldest months are February and January with an average of 39°F. The rainiest months in Iwamuro Onsen are July, December, January, and August, with each month seeing an average of 9 inches of rainfall.