Weather is probably a major factor when you plan your trip to Unzen, but bear in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually August and July, with an average temperature of 23°C, while the coldest months are January and February, with an average of 7°C. The rainiest months in Unzen are June, July, August and May, with each month seeing an average of 376 mm of rainfall.