Wine regions in the U.S. are usually classified according to a system developed at the University of California at Davis known as the Degree Day System. This is a system that compares the sum of the average number of degrees each day above 50ºF between April 1 and October 31 each year.
Region with less than 2500 degree days are classified as Region I. These tend to be coastal regions where Pinot Noir and white wines are most successful, equivalent to a range between the Mosel in Germany and Burgundy.
Vineyards classified between 2500–3000 degree days are classified as Region II, and are also often cooler areas close to the coast, roughly equivalent to the climate of Bordeaux at the low end and Piemonte at the high end.
Vineyards classified as 3000–3500 degree days are warmer Region III, and tend to produce Cabernet, Zinfandel, Syrah and dessert wines, roughly equivalent to Southern France or Central Italy, while those between 3500–4000 degree days are Region IV, and used mostly for fortified wines or lower quality table wines.
Those classified above 4000 degree days are Region V, and tend to come from the hottest part of the Central Valley.
The United States is a country with a relatively short wine producing history, and this has both positive and negative influences on the quality of the wine. Many of the vineyards are very recent, and have been planted using the best of modern grape growing technique in terms of canopy management and pruning and trellising systems, such as cordon training and vertical shoot positioning to maximize the amount of light reaching the canopy. In addition, growers are free to make use of techniques such as drip irrigation that have the potential to improve quality.
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