Four mountain ranges protect the winegrowing hills from cool and moist weather. In the north, this includes the Serra de Alvấo, the Serra de Padrela and the Serra de Bornes, which set the Douro region apart from the cooler climate of the Minho (Vinho Verde). Toward the west, the infamous Serra do Marấo, which rises to 1,400 m, serves as a reliable weather divide. In the summertime one frequently leaves the city of Porto in overcast, foggy and moist weather only to encounter the wonderful vista of the peaceful valley in glowing sunshine after coming past the many curves of the Marấo Pass. The heat in the Douro region is often insufferable and can reach temperatures between 40 and 50 degrees Celsius in the daytime, which is usually 15 to 25 degrees above the temperature of the relatively cool Porto. The highest temperatures are reached in the vicinity of the river and in the adjoining valleys on the right-hand bank.
In contrast, winter temperatures in the Douro region can be quite low (around 0 degrees Celsius) and in some exceptional years (2003) there will even be snowfall. Once travelers cross the Marấo toward the west, they usually encounter mild temperatures, soft sunshine and blossoming trees.
The Marão has an enormous significance as a weather divide. Even though the climate of the city of Porto on the Atlantic Coast is not suitable for viticulture, it offers the perfect cool and humid conditions to store wines for many years. Traditionally, the wine casks from the Douro region were brought to Porto (to be more precise: to Vila Nova de Gaia) every year in June before the beginning of the greatest heat to finish maturing in the lodges of the shippers. They were shipped downriver on sailboats ("barcos rabelos"), which today only anchor on the Douro at Vila Nova de Gaia as an open-air museum. (Almost) every port wine lodge polishes up its company boat only once a year: the traditional sailing regatta takes place on the day of the town's patron saint São João (June 24) and the employees of the port wine lodges prove their mettle in a sports competition.
