The climate in the Douro is characterised by variation; annual temperatures fluctuate between extremes. Winter temperatures often fall below freezing, and yet it commonly climbs above 40º C at the height of summer. These variations are more pronounced further east.
The dominant weather patterns arrive from the Atlantic in the west, and are forced up over the Serra do Marão hills. Rainfall is therefore largely dictated by altitude and proximity to the hills, with the effect that annual rainfall, concentrated mainly in November, December and January, shows remarkable differences across very small distances. On the higher land it can be as high as 1200 mm, and as a general rule rainfall decreases further inland. This corresponds to around 1000 mm at Régua, through 700 mm at Pinhão, and down to as low as 400 mm by the Spanish border. The practical effects of this are that the hotter, drier regions further to the east tend to produce smaller volumes of riper, richer and more concentrated wines.