Weather is probably a major factor when you plan your trip to Mersing, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually May and June with an average temp of 82°F, while the coldest months are January and February with an average of 79°F. The rainiest months in Mersing are December, November, January, and October, with each month seeing an average of 15 inches of rainfall.