You'll likely spend much of your trip indoors, but if you want to see more of the area, taking a look at local weather data can help you. The hottest months are usually August and July, with an average temperature of 20°C, while the coldest months are January and February, with an average of 4°C. The rainiest months in Rijeka are November, December, September and October, with each month seeing an average of 182 mm of rainfall.