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 30 degrees above zero. On average, Galveston has 15 sunny, 13 rainy, and 3 partly cloudy days in August. The rains are usually heavy. August as well as July, September, and December is considered the most uncomfortable time to visit Galveston.