Stavanger: Annual Weather Averages. July is the hottest month in Stavanger with an average temperature of 15°C (59°F) and the coldest is January at 1°C (34°F) with the most daily sunshine hours at 8 in June. The wettest month is October with an average of 110mm of rain. The best month to swim in the sea is in August when the average sea temperature is 16°C (61°F).