September in Cheyenne is the sunniest month of the year. In September, the weather in Cheyenne is typically warm. Average daily high temperature in September is about 21 degrees above zero. On average, Cheyenne has 16 sunny, 10 partly cloudy, and 4 rainy days in September. The rains are usually moderate. September as well as June is considered the most comfortable time to visit Cheyenne.