If you’re looking forward to lots of outings with your partner, take a look at these details about year-round temperatures in Nagano: The hottest months are usually August and July, with an average temperature of 21°C, while the coldest months are January and February, with an average of 0°C. The rainiest months in Nagano are July, December, January and August, with each month seeing an average of 255 mm of rainfall.