August in Galveston is the hottest month of the year. In August, the weather in Galveston is typically hot. Average daily high temperature in August is about 32 degrees above zero. On average, Galveston has 16 rainy, 10 sunny, 4 partly cloudy, and 1 cloudy day in August. The rains are usually heavy. August as well as June, July, and September is considered the most uncomfortable time to visit Galveston.