If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Kusatsu: The hottest months are usually August and July with an average temp of 67°F, while the coldest months are January and February with an average of 31°F. The rainiest months in Kusatsu are July, August, June, and September, with each month seeing an average of 10 inches of rainfall.