If you're hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Sanbu: The hottest months are usually July and August with an average temp of 83°F, while the coldest months are January and December with an average of 63°F. The rainiest months in Sanbu are August, June, May, and September, with each month seeing an average of 12 inches of rainfall.