If you're looking forward to lots of outings with your partner, take a look at these details about year-round temperatures in Dumai: The hottest months are usually May and June, with an average temperature of 27°C, while the coldest months are January and December, with an average of 26°C. The rainiest months in Dumai are November, December, October and April, with each month seeing an average of 346 mm of rainfall.