Climate

Climate is the most immediately obvious of all the environmental factors which affects vines, and includes temperature, sunshine, rainfall, frost and the impact of wind.

Vines are dormant below about 50°F (10°C), and ripening occurs only above about 63°F (17°C). Vine function diminishes above 75°F (24°C), and the vine may shut down entirely at temperatures higher than about 90°F (32°C). Sunshine is related to heat, but is not the same. Photosynthesis requires sunlight, not heat, so hot cloudy climates are not ideal for producing wine grapes. Sunny days well into fall are crucial, as harvest may take place late in September or even into October (March or April in the southern hemisphere).

A vine needs about 20 inches (500 mm) of water per year in cool climates, rising to about 30 inches (750 mm) in the warmest regions, both for general functioning and to promote photosynthesis. In traditional regions where irrigation is banned, this may mean that it is difficult to obtain a good vintage in drought years (though most of the places that refuse to irrigate are temperate with a good rainfall). Where irrigation is practiced, the level of rainfall becomes less important-as long as dam water levels or river flow are maintained.

Frost affects the quantity of wine more than the quality. Winter frosts are rarely a problem, but a frost late in spring can literally nip a crop in the bud by burning off the spring shoots. In 1991, the yields in Touraine in the Loire valley were about 10 percent of normal because a severe frost that struck on April 21-22 destroyed most shoots. The impact of frosts can be mitigated by good site selection, or by artificial means. The most basic of these is using hot braziers in the vineyards to raise the temperature, while one of the more technologically advanced ways is aspersion-spraying the shoots with water, which freezes and, paradoxically, insulates them against the worst of the cold.

Wind may cool the vines, or burn them when the wind is warm, and at its worst can rip leaves off. While it is not a general problem, it may be recurrent in specific regions such as the Rhone valley, where the Mistral can whip down towards the Mediterranean, stressing the vines and impeding ripening. Winds can also be beneficial in a warm climate, as in the Hunter region in Australia, where the valley funnels in sea breezes to moderate the heat of the sun.

Comparing climates between different wine regions is fraught with difficulty, and has to take account of temperature and temperature variability, sunlight and sun angle, rainfall, wind and various other factors. One can roughly categorize wine regions in four ways-maritime, continental, Mediterranean and hot inland.

From this, it becomes apparent that Bordeaux and the Rheingau have broadly similar summer temperatures (the former is a bit hotter, and has marginally more sunshine), but the continental climate is markedly colder in winter. The Mediterranean and hot inland climates are both warmer in summer, and noticeably warmer in winter. The maritime climate, with its proximity to the ocean, is the wettest, but the Mediterranean is also quite wet (though it is in fact rather drier in summer, which is why it has the highest number of sunshine hours). The inland region has the least rainfall, and irrigation is essential here.

It is impossible to use climatic modeling to predict exactly what varieties will be used, and which wines will be made, but the wines of each of these nominated regions shows something about their climate:

An important distinction must be made between the climate of a wine region and its weather. The climate is the average of all the weather factors over a long period. It allows one to predict what is likely to happen to a vineyard planted there and, therefore, is a key indicator of site selection. The weather is what happens in a particular year, and it will inevitably deviate from the climatic norms. Thus, 1987 in Bordeaux was wetter than normal, especially in autumn, so that cabernet sauvignon grapes did not ripen properly. On the other hand, 1990 in the same region was warmer and sunnier than usual, resulting in an early and large crop of high-quality grapes. That is how the impact of weather leads to vintage variation, though it is more of an issue in marginal climates. In warmer regions, there is likely to be less variation from one year to the next, with fewer poor crops and fewer pronounced high-quality vintages. Hail is an unpredictable weather condition that can devastate a vineyard-ripping leaves off and puncturing grapes in a matter of minutes, often on quite a localized basis.