The Winkler scale, sometimes known as the heat summation method, is a technique for classifying the climate of wine growing regions. In the system, geographical areas are divided into five climate regions based on temperature, known as Regions I–V. The system was developed at the University of California, Davis by A. J. Winkler and Maynard Amerine.
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The system is based on the hypothesis that grapevines do not grow if the temperature is below 50 °F (10 °C). Days in the growing region (assumed under the system to be April 1 through October 31 in the Northern Hemisphere; October 1 through April 30 in the Southern Hemisphere) are assigned degree days according to the amount that the day's average temperature exceeds this threshold; one degree day per degree Fahrenheit over 50 °F. In places where SI units are preferred, degrees Celsius over 10 °C may be used, but should be multiplied by 1.8 to convert to Fahrenheit degree days for the following list. All days in the locale are then added up, with the sum used to determine the region's classification as follows:
The system is used officially in California, and other United States growing regions such as Oregon and Washington. It is less widely used elsewhere; however degree days can be computed for any location for which detailed climate data is available.
Different varieties of grapes are generally considered to best thrive in certain climate regions. Region I, the coolest, is similar to Côte d'Or and Champagne, the Rhine, or the Willamette Valley in Oregon; it is well-suited to growing Chardonnay, Pinot Noir, or Riesling. Region II is similar to Bordeaux. Suitable varietals include the Region I wines, plus Cabernet Sauvignon, Sauvignon Blanc, Cabernet Franc and Merlot. Region III is similar to the Rhône, and is best suited to Sauvignon Blanc, Sémillon, Syrah or Zinfandel. Regions IV and V do not correspond to any growing region in France; the former is similar to Spain, and is well-suited to Port and Barbera. Region V is similar to North Africa, and is best suited to wines like Muscat (grape and wine) or varieties like Verdelho.
California has growing regions which lie in all five regions; from Mendocino and Sonoma in the north (which lie in regions I-III) to the San Joaquin Valley and points south, which lie in regions IV and V.
The climate regions of California only describe one aspect of an area's climate—mean daily temperature. Many other important factors which contribute to a region's suitability for viticulture (and its terroir) are exluded; among them sun exposure, latitude, precipitation, soil conditions, the likeliness of extreme weather which might damage grapevines, and pollution. The climate regions are also macroscopic in nature; there is often a wide variety of microclimates in a given geographical area, and a region which has marginal grape-growing weather overall may have microclimates which produce excellent grapes. A notable example is the Willamette Valley (firmly within region I), which was long regarded as too cold and wet to grow grapes; yet has vineyards planted numerous south-facing hills in the rain shadow of the Coast range which produce world-class Pinot Noir and many other excellent wines.
More complex climate indices have been introduced to address perceived shortcomings in the Winkler scale.