Weather will likely play an important role when you plan your trip to Dumai, but bear 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 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.