Temperature and sunlight are central in determining wine quality and style, because sunlight enables photosynthesis and temperature determines respiration. In hotter, sunnier climates, grapes produce wines with fuller body, higher alcohol and lower acidity. Temperature after veraison, or the moment when grapes change color and begin to ripen, is particularly important since insufficient temperature during ripening will produce wine that is lean, austere and lacking in fruit and body. For example, Cabernet Sauvignon ripens fairly late and in cooler climates it can produce thin, acidic wines without proper fruit development.
In general, the more consistent the temperature, the more consistently grapes will ripen. This lack of fluctuation is referred to as equability. Two factors that aid equability are proximity to large bodies of water and thermal inversion created by the slope of the hills on which the vineyards are planted. Finally, the temperature swing between winter and summer also influences quality. Late-ripening grapes, such as Cabernet Sauvignon, do not tolerate a sudden drop in autumn temperature very well and can produce herbaceous, unripe grapes as a result.
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The amount of rain and the time of rainfall during the growing season are important elements of climate. Rainfall is particularly important in areas where irrigation is not allowed, as is the case in much of Europe. In general, cool climates require at least 500 mm/yr of rain to produce wine grapes, while warmer climates need 600–750 mm/yr. Rainfall is less important in regions where irrigation is permitted, but ill-timed rain can still ruin a harvest or have a dramatically adverse effect on the quality of the wine.