You'll likely spend much of your trip indoors, but if you want to see more of the area, looking at local weather trends is a good idea. 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 1°C. The rainiest months in Tateyama are July, December, September and August, with each month seeing an average of 299 mm of rainfall.