Weather is probably a major factor when you plan your trip to Kosaka, and keep 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 temp of 68°F, while the coldest months are February and January with an average of 29°F. The rainiest months in Kosaka are July, August, September, and October, with each month seeing an average of 9 inches of rainfall.