August in Merrillville is the sunniest month of the year. In August, the weather in Merrillville is typically warm. Average daily high temperature in August is about 27 degrees above zero. On average, Merrillville has 20 sunny, 7 partly cloudy, and 4 rainy days in August. The rains are usually light. August as well as September is considered the most comfortable time to visit Merrillville.