The climate in Portugal varies from region to region and it is influenced by the relief, latitude and Ocean proximity.
The north and the countryside regions either because of a higher relief or because they are far from the Atlantic Ocean, they can reach more extreme temperatures than the coastal and south regions, which are flatter and so they have milder temperatures.
Lisbon has a mild climate. So in Winter times the average temperature is 12º C/54ªF and in Summer times the average temperature is25 ªC /77ª F. There is always a breeze coming from the Atlantic Ocean. So, even in a Summer night you should bring a jersey.
The hottest period of the year is July and August and the coldest is December and January. The rainy season is between January and April.
Besides the jersey for Summer nights, you should bring comfortable shoes to walk over the Portuguese cobbled stone pavement. It is also necessary to cover your knees and shoulders to go inside some churches. Ladies that have some scarf can use it over their shoulders.