If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Niihama: The hottest months are usually August and July with an average temp of 75°F, while the coldest months are January and February with an average of 43°F. The rainiest months in Niihama are June, July, September, and August, with each month seeing an average of 11 inches of rainfall.