August in Fairfax is the cloudiest month of the year. In August, the weather in Fairfax is typically hot. Average daily high temperature in August is about 30 degrees above zero. On average, Fairfax has 12 rainy, 8 sunny, 8 partly cloudy, and 3 cloudy days in August. The rains are usually heavy. August as well as January, February, March, and July is considered the most uncomfortable time to visit Fairfax.