In September, the weather in Longyearbyen is typically cold. Average daily high temperature in September is about 1 degree above zero. On average, Longyearbyen has 15 cloudy, 7 rainy, 5 partly cloudy, 2 sunny, and 1 snowy day in September. The rains are usually moderate. September as well as January, February, March, and April is considered the most uncomfortable time to visit Longyearbyen.