If you're looking forward to lots of outings with your partner, take a look at these details about year-round temperatures in Chengmai: The hottest months are usually June and May, with an average temperature of 28°C, while the coldest months are January and December, with an average of 20°C. The rainiest months in Chengmai are August, July, September and May, with each month seeing an average of 264 mm of rainfall.