In August, the weather in Merthyr Tydfil is typically warm. Average daily high temperature in August is about 16 degrees above zero. On average, Merthyr Tydfil has 24 rainy, 4 sunny, and 3 cloudy days in August. The rains are usually heavy. August as well as January, February, March, and April is considered the most uncomfortable time to visit Merthyr Tydfil.