If you start your trip from Lisbon in August, you'll walk in pretty warm weather during the day, around 26-32°C. The nights are cooler, around 15-18°C. It'll mostly be sunny with some clouds and a little bit of rain sometimes in Portugal.
When you get to the Galicia area in Spain, towards Tui and Santiago de Compostela, it gets a bit cooler. Daytime temperatures are about 24-29°C and nighttime temperatures are around 15-17°C. It's often sunny but there are also many clouds and some rain. As you get closer to Santiago, there's more rain and it can sometimes be quite heavy. But the weather can change suddenly. In general, if you go in August, it's a mix of sunshine and clouds, it can get fairly hot, and there might be some mild to moderate rain.
Zoom in and click on town for more info.
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