When you walk the Senda Litoral from Porto to Santiago de Compostela, it's generally not too hot or cold. Daily peak temperatures are usually between 15-18°C, going down to 9-14°C.
In the north of Portugal, from Porto to Caminha, it often rains. Sometimes it's just a bit of rain, other times it's heavy showers. It also can be sunny and a bit cloudy. As you get further along the coast, it gets a bit colder but not by much.
Once you cross into Galicia in Spain, expect more rainfall. Especially in the towns of Saians and Redondela, it rains quite heavily, much more than in Portugal. The temperatures don't change much.
As you come to the end of the route at Santiago de Compostela, you'll see a lot of cloudy and overcast weather. Remember it can rain heavily at times. But don't worry, there are also days with some sunshine.
All across the Senda Litoral, it usually rains during the journey, mixed with sunny and partly cloudy days. Even though weather changes a bit from one region to another, the temperature in general doesn't really change much.
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