September in Cornwall is the sunniest month of the year. In September, the weather in Cornwall is typically warm. Average daily high temperature in September is about 20 degrees above zero. On average, Cornwall has 15 sunny, 7 rainy, 7 partly cloudy, and 1 cloudy day in September. The rains are usually moderate. September as well as {{month_nom: February}}, and {{month_nom: February}} is considered the most comfortable time to visit Cornwall.