If you’re looking forward to lots of outings with your partner, take a look at these details about year-round temperatures in Surabaya: The hottest months are usually October and September, with an average temperature of 29°C, while the coldest months are July and August, with an average of 28°C. The rainiest months in Surabaya are January, February, December and March, with each month seeing an average of 309 mm of rainfall.