Taking the local weather in Koshu into account is an important factor for planning your visit, especially if you plan to spend your trip by the pool. The hottest months are usually August and July, with an average temperature of 19°C, while the coldest months are January and February, with an average of 0°C. The rainiest months in Koshu are July, September, October and June, with each month seeing an average of 236 mm of rainfall.