If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Chengmai: The hottest months are usually June and May with an average temp of 82°F, while the coldest months are January and December with an average of 68°F. The rainiest months in Chengmai are August, July, September, and May, with each month seeing an average of 10 inches of rainfall.