September in Reykyavik is the rainiest month of the year. In September, the weather in Reykyavik is typically warm. Average daily high temperature in September is about 10 degrees above zero. On average, Reykyavik has 18 rainy, 4 sunny, 4 cloudy, and 4 partly cloudy days in September. The rains are usually heavy. September as well as January, February, March, and April is considered the most uncomfortable time to visit Reykyavik.