Weather is probably a major factor when you plan your trip to Cheonan, 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 73°F, while the coldest months are January and December with an average of 34°F. The rainiest months in Cheonan are July, August, September, and June, with each month seeing an average of 9 inches of rainfall.