July in Viana do Castelo is the sunniest month of the year. In July, the weather in Viana do Castelo is typically warm. Average daily high temperature in July is about 22 degrees above zero. On average, Viana do Castelo has 29 sunny and 2 rainy days in July. The rains are usually light. July as well as August and September is considered the most comfortable time to visit Viana do Castelo.