Planning for the weather is a good way to make your trip to Sonohara relaxing and enjoyable, especially when you're splurging. The hottest months are usually August and July, with an average temperature of 20°C, while the coldest months are January and February, with an average of 1°C. The rainiest months in Sonohara are July, June, September and August, with each month seeing an average of 337 mm of rainfall.