Planning for the weather is a good way to make your trip to Mong Cai relaxing and enjoyable, especially when you're splurging. The hottest months are usually July and June with an average temp of 83°F, while the coldest months are January and December with an average of 63°F. The rainiest months in Mong Cai are August, July, June, and September, with each month seeing an average of 13 inches of rainfall.