Planning for the weather is a good way to make your trip to Viana do Castelo relaxing and enjoyable, especially when you’re splurging. The hottest months are usually August and July, with an average temperature of 19°C, while the coldest months are February and January, with an average of 11°C. The rainiest months in Viana do Castelo are October, December, November and January, with each month seeing an average of 222 mm of rainfall.