If you're looking forward to lots of outings with your partner, check out these details about year-round temperatures in Nanseien: The hottest months are usually July and August with an average temp of 83°F, while the coldest months are January and February with an average of 68°F. The rainiest months in Nanseien are August, May, September, and June, with each month seeing an average of 11 inches of rainfall.