Climate and Wine Quality: Sunlight

Contrary to common perception, cool viticultural climates are probably more limited by their low temperatures than by lack of sunlight. Heat as summed over the season determines which grape varieties, if any, reach a satisfactory degree of ripeness. Sunlight duration acts mainly by controlling sugar in grapes and therefore potential wine alcohol content at a given stage of physiological ripening. In practice, however, the relative contributions of sunlight and temperature are hard to distinguish because low temperatures and low sunlight hours tend to go together. Poor seasons in cool climates are usually both sunless and cold.