Climate is determined by the amount of sunshine, heat and rain that a region receives on a yearly basis. Weather is the vintage variation from year to year, and also includes one-time events such as frost or hail.
 Temperature and sunlight are central issues in wine quality and style, since sunlight causes photosynthesis, while temperature determines respiration. In hotter, sunnier climates, grapes produce wines with fuller body, higher alcohol and lower acidity. Temperature after veraison is particularly important, since insufficient temperature during ripening will give wine that is lean, austere, lacking fruit and body. For example, Cabernet is a fairly late ripener, and in cooler climates can produce thin, acidic wines without proper fruit development. Each grape is different, however, and what is not enough for Cabernet might well be too much for Pinot Noir.
In general, the more even and constant the temperature is, the better the grapes will ripen. This lack of fluctuation is referred to as equability. Two factors that aid equability are the presence of large bodies of water and the thermal inversion created by the slope of the hills on which the vineyards are planted (see topography, above). Finally, the swing between winter and summer (continentality) also influences quality, because late ripening grapes such as Cabernet do not tolerate a sudden drop in autumn temperature very well, and can produce herbaceous, unripe wine. Bud burst is also more even and complete after a solid dormant period.
Sunlight is also an important factor in wine quality. Ultimately, grapes ripen because of photosynthesis, which is the binding together of water and CO2 from the atmosphere to form starch. This is accomplished with the help of light energy from the sun absorbed by chlorophyll. Ultimately, a vine needs both sunlight and warm temperatures to ripen fruit effectively.
There is another function that takes place at the same time as ripening, however, called respiration. During respiration, oxygen combines with the sugar from the vine and with other compounds, including amino acids and organic acids, and CO2 is released. As Richard Smart observes in the Oxford Companion to Wine, respiration is the opposite of photosynthesis, and vines respire more sugar in warmer weather, although the rate of photosynthesis remains the same.
 Rainfall is also an important element of climate, both the quantities received as well as the period of the year when it tends to fall. This is particularly important in areas where irrigation is not allowed, as in much of Europe. In general, cool climates need at least 500 mm/yr of rain to produce wine grapes, and warmer climates need 600 – 750 mm/yr. It is less important where irrigation is permitted, but ill-timed rain can still ruin a harvest, or at least make it much more difficult to sell. The importance of water and thus rain is explained above in the section on soil.
The weather (or yearly variation in climate) is a fixation of wine lovers. Warm, mild temperatures at bud break can give the vines an early start, while the same type of weather at flowering can lead to a bigger crop because pollination of the vines will be more regular. As seen above, warmth and sunshine during ripening give richer, fatter wines. It is important to any winemaker, however, to maintain balance, and if acidity falls too rapidly at the end of ripening and before phenolic ripeness is reached, it can lead to wines with moderate alcohol and body but lacking in fruit and showing hard, astringent tannins. Finally, dry weather at harvest allows the vine to ripen and helps the vine avoid rot and dilution. Late season hailstorms can devastate ripe grapes, as can ill-timed frosts.
The date of first and last frost also play a role is determining quality. Some grapes bud early, and are therefore ill-suited to climates prone to late frosts. Others ripen late, and ripen with difficulty in regions with early frosts in the fall. Particularly in the spring when the buds are very tender, frost damage can severely limit yield and quality.
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Wine Regions of the World
