May in Keflavík is the sunniest month of the year. In May, the weather in Keflavík is typically cold. Average daily high temperature in May is about 8 degrees above zero. On average, Keflavík has 17 rainy, 7 sunny, 4 cloudy, and 3 partly 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 Keflavík.