May in Selfoss is the sunniest month of the year. In May, the weather in Selfoss is typically cold. Average daily high temperature in May is about 8 degrees above zero. On average, Selfoss has 16 rainy, 6 sunny, 6 partly cloudy, and 3 cloudy days in May. The rains are usually heavy. May as well as January, February, March, and April is considered the most uncomfortable time to visit Selfoss.