August in Nantucket is the sunniest and the warmest month of the year. In August, the weather in Nantucket is typically warm. Average daily high temperature in August is about 22 degrees above zero. On average, Nantucket has 18 sunny, 8 partly cloudy, and 5 rainy days in August. The rains are usually heavy. August as well as July and September is considered the most comfortable time to visit Nantucket.