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The Winkler Index, sometimes known as the Winkler Scale or Winkler Regions, is a technique for classifying the
climate Climate is the long-term weather pattern in a region, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteoro ...
of
wine Wine is an alcoholic drink made from Fermentation in winemaking, fermented fruit. Yeast in winemaking, Yeast consumes the sugar in the fruit and converts it to ethanol and carbon dioxide, releasing heat in the process. Wine is most often made f ...
growing region A growing region, also known as a farming region or agricultural region, refers to a geographic area characterised by specific climate factors, soil conditions and agricultural practices that are favourable for the cultivation and production of ...
s based on heat summation or
growing degree-day Growing degree days (GDD), also called growing degree units (GDUs), are a heuristic tool in phenology. GDD are a measure of heat accumulation used by horticulturists, gardeners, and farmers to predict plant and animal development rates such as the ...
s. In the system, geographical areas are divided into five climate regions based on temperature converted to
growing degree-day Growing degree days (GDD), also called growing degree units (GDUs), are a heuristic tool in phenology. GDD are a measure of heat accumulation used by horticulturists, gardeners, and farmers to predict plant and animal development rates such as the ...
s, and is commonly known as Regions I–V (see below). The system was developed at the
University of California, Davis The University of California, Davis (UC Davis, UCD, or Davis) is a Public university, public Land-grant university, land-grant research university in Davis, California, United States. It is the northernmost of the ten campuses of the University ...
by
A. J. Winkler Albert Julius Winkler (4 March 1894, in The Grove, Texas – 29 August 1989, in Woodland, California) was an American professor of viticulture and one of its leading authorities. His name is famous for the Winkler index, developed with Maynard ...
and Maynard Amerine.


The system

The system is based on both the hypothesis and observations that
grapevines ''Vitis'' (grapevine) is a genus of 81 accepted species of vining plants in the flowering plant family Vitaceae. The genus consists of species predominantly from the Northern Hemisphere. It is economically important as the source of grapes, bot ...
do not grow if the temperature is below 50 °F (10 °C). Each day during the growing season are assigned
growing degree-day Growing degree days (GDD), also called growing degree units (GDUs), are a heuristic tool in phenology. GDD are a measure of heat accumulation used by horticulturists, gardeners, and farmers to predict plant and animal development rates such as the ...
s according to the amount that the day's average temperature exceeds this threshold. This is assumed under the system to be April 1 through October 31 in the Northern Hemisphere, October 1 through April 30 in the Southern Hemisphere. One degree day per degree
Fahrenheit The Fahrenheit scale () is a scale of temperature, temperature scale based on one proposed in 1724 by the German-Polish physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accou ...
over 50 °F, or with SI units, degrees
Celsius The degree Celsius is the unit of temperature on the Celsius temperature scale "Celsius temperature scale, also called centigrade temperature scale, scale based on 0 ° for the melting point of water and 100 ° for the boiling point ...
over 10 °C is used. All days during the growing season are then added up, all negative values are set to zero, with the sum of the growing degree-days used to determine the region's classification in the original Winkler index as follows: The system was originally developed for and is used officially in
California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...
and was based on the general ripening capabilities and wine styles that can be achieved in the climate due to heat accumulation (growing degree-days). The general ripening capabilities include hybrid grape varieties through early season, mid-season, and late season ripening ''V. Vinifera'' and even
table grape Table grapes are grapes intended for consumption as fresh fruit, as opposed to grapes grown for wine production, Juicing, juice production, jelly and jam making, or for drying into raisins. ''Vitis vinifera'' table grapes can be in the form of e ...
s in the warmest areas of Region V. The general wine styles include lighter, more subtle wines with lower alcohol and brighter fruit aromas and flavors, including
Champagne Champagne (; ) is a sparkling wine originated and produced in the Champagne wine region of France under the rules of the appellation, which demand specific vineyard practices, sourcing of grapes exclusively from designated places within it, spe ...
and other sparkling wines, found in cooler climates (Regions Ia, Ib, II and lower III) to bolder, bigger wines often with higher alcohol and lush, darker fruit aromas and flavors that are found in warmer climates (Region III, IV and V). Region V was stated as also having a tendency to be more suitable to higher production wines,
Sherry Sherry ( ) is a fortified wine produced from white grapes grown around the city of Jerez de la Frontera in Andalusia, Spain. Sherry is a drink produced in a variety of styles made primarily from the Palomino grape, ranging from light versio ...
and other
fortified wine Fortified wine is a wine to which a distilled spirit, usually brandy, has been added. In the course of some centuries, winemakers have developed many different styles of fortified wine, including port, sherry, madeira, Marsala, Command ...
s. One issue with the original work done by Amerine and Winkler was that it did not specify a lower class limit for Region I (originally 2500 or less) or an upper class limit for Region V (originally 4000 or greater). Subsequent research using high resolution spatial climate data identified these limits for California, Oregon, Washington and Idaho, along with Australia. The results provided a lower bound to Region I of 1500 °F units (850 °C units) and an upper bound to Region V of 4900 °F units (2700 °C units). Additional research in other wine regions found that Region I was best divided into a Region Ia (very early ripening varieties, mostly hybrid grapes) and Region Ib (early ripening varieties, mostly ''V. Vinifera''). The Winkler Index is also widely used in many other growing regions in the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, such as
Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...
and
Washington Washington most commonly refers to: * George Washington (1732–1799), the first president of the United States * Washington (state), a state in the Pacific Northwest of the United States * Washington, D.C., the capital of the United States ** A ...
, along with
Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
, South America,
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
,
New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
,
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...
, and Europe. However, it is less widely used in Europe where the
Huglin index Pierre Huglin developed a bioclimatic heat index for vineyards, the Huglin heat sum index (or ''after Huglin'' respectively -warmth index or short Huglin index,) in which the temperature sum over the temperature threshold of 10 °C is calcula ...
is favored. The Huglin index uses a similar formula but gives more weight to maximum temperatures and uses an adjustment for longer day lengths found at higher latitudes. It is also functionally similar to growing season average temperatures (simple average of temperatures across the seven month
growing season A season is a division of the year marked by changes in weather, ecology, and the amount of daylight. The growing season is that portion of the year in which local conditions (i.e. rainfall, temperature, daylight) permit normal plant growth. Whi ...
).


Application

The table below provides examples of the ripening and wine style concept used in the application of the Winkler Index for numerous wine regions globally. Region Ia are the coolest areas with known regions including
Champagne Champagne (; ) is a sparkling wine originated and produced in the Champagne wine region of France under the rules of the appellation, which demand specific vineyard practices, sourcing of grapes exclusively from designated places within it, spe ...
,
Central Otago Central Otago is an area located in the inland part of the Otago region in the South Island of New Zealand. The motto for the area is "A World of Difference". The area is dominated by mountain ranges and the upper reaches of the Clutha River ...
, and
Valais Valais ( , ; ), more formally, the Canton of Valais or Wallis, is one of the cantons of Switzerland, 26 cantons forming the Switzerland, Swiss Confederation. It is composed of thirteen districts and its capital and largest city is Sion, Switzer ...
. Region Ia also includes numerous newer regions growing grapes and making wine including
southern England Southern England, also known as the South of England or the South, is a sub-national part of England. Officially, it is made up of the southern, south-western and part of the eastern parts of England, consisting of the statistical regions of ...
, areas in northern
Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...
,
Nova Scotia Nova Scotia is a Provinces and territories of Canada, province of Canada, located on its east coast. It is one of the three Maritime Canada, Maritime provinces and Population of Canada by province and territory, most populous province in Atlan ...
, and southern areas of
Chile Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...
and
Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...
. Region Ia areas ripen a range of
hybrid grape Hybrid grapes are grape varieties that are the product of a Hybrid (biology), crossing of two or more ''Vitis'' species. This is in contrast to crossings between grape varieties of the same species, typically ''Vitis vinifera'', the European grape ...
s and some very early ripening '' V. Vinifera.'' Region Ib is slightly warmer, can ripen early varieties such as
Chardonnay Chardonnay (, ; ) is a green-skinned grape variety used in the production of white wine. The variety originated in the Burgundy wine region of eastern France, but is now grown wherever wine is produced, from England to New Zealand. For new a ...
,
Pinot noir Pinot noir (), also known as Pinot nero, is a red-wine grape variety of the species ''Vitis vinifera''. The name also refers to wines created predominantly from Pinot noir grapes. The name is derived from the French language, French words fo ...
,
Sauvignon blanc Sauvignon blanc () is a green-skinned grape variety that originates from the city of Bordeaux in France. The grape most likely gets its name from the French words ''sauvage'' ("wild") and ''blanc'' ("white") due to its early origins as an ind ...
or
Riesling Riesling ( , ) is a white grape variety that originated in the Rhine region. Riesling is an aromatic grape variety displaying flowery, almost perfumed, aromas as well as high acidity. It is used to make dry, semi-sweet, sweet, and sparkling ...
with characteristic locations within the
Rhine The Rhine ( ) is one of the List of rivers of Europe, major rivers in Europe. The river begins in the Swiss canton of Graubünden in the southeastern Swiss Alps. It forms part of the Swiss-Liechtenstein border, then part of the Austria–Swit ...
and Mosel valleys,
Burgundy Burgundy ( ; ; Burgundian: ''Bregogne'') is a historical territory and former administrative region and province of east-central France. The province was once home to the Dukes of Burgundy from the early 11th until the late 15th century. ...
and the
Loire Valley The Loire Valley (, ), spanning , is a valley located in the middle stretch of the Loire river in central France, in both the administrative regions Pays de la Loire and Centre-Val de Loire. The area of the Loire Valley comprises about . It is r ...
, or the
Willamette Valley The Willamette Valley ( ) is a valley in Oregon, in the Pacific Northwest region of the United States. The Willamette River flows the entire length of the valley and is surrounded by mountains on three sides: the Cascade Range to the east, the ...
in
Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...
as good examples. Region II includes cooler locations within areas such as
Bordeaux Bordeaux ( ; ; Gascon language, Gascon ; ) is a city on the river Garonne in the Gironde Departments of France, department, southwestern France. A port city, it is the capital of the Nouvelle-Aquitaine region, as well as the Prefectures in F ...
, Coonawarra, and Valle de Curicó in
Chile Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...
. Warmer areas in these wine regions fall in a Winkler Region III as do much of the Northern Rhône, Rioja,
Umbria Umbria ( ; ) is a Regions of Italy, region of central Italy. It includes Lake Trasimeno and Cascata delle Marmore, Marmore Falls, and is crossed by the Tiber. It is the only landlocked region on the Italian Peninsula, Apennine Peninsula. The re ...
, and the
Margaret River The Margaret River is a river in southwest Australia, southwest Western Australia. In a small catchment, it is the eponym of the town and tourist region of Margaret River, Western Australia, Margaret River. The river arises from a catchment ...
. Region IV includes portions of the
Napa Valley Napa Valley is an American Viticultural Area (AVA) in Napa County, California. The area was established by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) on February 27, 1981, after a 1978 petition submitted by the Napa Valley Vin ...
,
Stellenbosch Stellenbosch (; )A Universal Pronouncing Gazetteer.
Thomas Baldwin ...
,
Corsica Corsica ( , , ; ; ) is an island in the Mediterranean Sea and one of the Regions of France, 18 regions of France. It is the List of islands in the Mediterranean#By area, fourth-largest island in the Mediterranean and lies southeast of the Metro ...
,
Tuscany Tuscany ( ; ) is a Regions of Italy, region in central Italy with an area of about and a population of 3,660,834 inhabitants as of 2025. The capital city is Florence. Tuscany is known for its landscapes, history, artistic legacy, and its in ...
, and
Alentejo Alentejo ( , , ) is a geographical, historical, and cultural region of south–central and southern Portugal. In Portuguese, its name means "beyond the Tagus" (). Alentejo includes the regions of Alto Alentejo Province, Alto Alentejo and Bai ...
where the warmer climates allow for the ripening of later varieties such as
Cabernet sauvignon Cabernet Sauvignon () is one of the world's most widely recognized red wine grape varieties. It is grown in nearly every major wine producing country among a diverse spectrum of climates from Australia and British Columbia, Canada to Lebano ...
,
Sangiovese Sangiovese is a red Italian wine grape variety that derives its name from the Latin , "blood of Jove, Jupiter". Sangiovese Grosso, used for traditionally powerful and slow maturing red wines, is primarily grown in the central regions of Italy ...
, and
Syrah Syrah (), also known as Shiraz, is a dark-skinned grape variety grown throughout the world and used primarily to produce red wine. In 1999, Syrah was found to be the offspring of two obscure grapes from southeastern France, Dureza and Mondeuse ...
. The warmest areas are found in Region V and include areas in the
central valley of California The Central Valley is a broad, elongated, flat valley that dominates the interior of California, United States. It is wide and runs approximately from north-northwest to south-southeast, inland from and parallel to the Pacific coast. It cover ...
, inland
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
and wine producing regions in
Morocco Morocco, officially the Kingdom of Morocco, is a country in the Maghreb region of North Africa. It has coastlines on the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to Algeria–Morocc ...
,
Madeira Madeira ( ; ), officially the Autonomous Region of Madeira (), is an autonomous Regions of Portugal, autonomous region of Portugal. It is an archipelago situated in the North Atlantic Ocean, in the region of Macaronesia, just under north of ...
,
Apulia Apulia ( ), also known by its Italian language, Italian name Puglia (), is a Regions of Italy, region of Italy, located in the Southern Italy, southern peninsular section of the country, bordering the Adriatic Sea to the east, the Strait of Ot ...
, and
Jerez Jerez de la Frontera () or simply Jerez, also cited in old English-language sources as , is a city and municipality in the province of Cádiz in the autonomous community of Andalusia, Spain. Located in southwestern Iberia, it lies on the Campi� ...
.


Issues and limitations

There are numerous issues and limitations associated with the use of growing degree-days. First, the Winkler index and its classification of climate regions by growing degree-days 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 (; ; from ''terre'', ) is a French language, French term used to describe the environmental factors that affect a crop's phenotype, including unique environment contexts, farming practices and a crop's specific growth habitat. Collectively, th ...
) are excluded; among them sun exposure,
latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...
,
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...
, soil conditions, and the risk of extreme weather which might damage grapevines (e.g., winter freezes, spring and fall frosts, hail, etc.). As originally developed the climates of California were defined for relatively large areas using only one or two climate stations. This macroscale approach will invariably not capture the microscale influences that are an important aspect of growing any crop. To address these issues research has been increasingly using spatial climate data to better depict within region and even within vineyard differences in climate and therefore ripening and wine style potential. To create spatially appropriate climate data, numerous stations and/or sensors are used to collect data which can then be interpolated over the landscape due to known interactions with elevation, aspect, slope, and distance to the coast or other water bodies using
Geographic Information Systems A geographic information system (GIS) consists of integrated computer hardware and software that store, manage, analyze, edit, output, and visualize geographic data. Much of this often happens within a spatial database; however, this is not ...
(GIS). Instead of depicting a region as all one Winkler region (
Napa Valley AVA Napa Valley is an American Viticultural Area (AVA) in Napa County, California. The area was established by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) on February 27, 1981, after a 1978 petition submitted by the Napa Valley Vin ...
being a Region III for example), spatial data summaries show the Napa Valley having a full range of Winkler regions, 12% a Region II, 56% a Region III, and 30% a Region IV (whereas the table above shows one station in Napa, St. Helena as being a Region IV). Other significant differences exist depending on the time period of the data and formula used for calculating growing degree-days. First, to be comparable growing degree-day numbers from various sources need to come from the same time period. Due to both a variable climate and
climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
, a comparison of a ten-year period from the 1970s and the 2000s would be inappropriate as the variation and trends over time would make them incomparable. A sufficient time period is suggested to allow the averaging to smooth out some of the variability. The standard time period in use is the
climatological normal Climatological normal or climate normal (CN) is a 30-year average of a weather variable for a given time of year. Monthly averaged data can be very problematic as it can underestimate heat accumulation during the first and last months of the growing season. Therefore, it is paramount that one know the time period that the growing degree-day values are calculated from so as to be comparable. The Winkler index uses the standard method of calculating growing degree-days in viticulture and is based on using a base temperature of 50 °F (10 °C) with no upper temperature cut-off. The first issue is that 50 °F (10 °C) is not likely the best base temperature even though it is the most commonly used value. Even the early research on this topic stressed that the base temperature threshold for accumulation for early versus late budding varieties is likely strongly cultivar specific. Various research worldwide has pointed to base temperatures ranging from 39 to 45 °F (4 to 7 °C), but there has been little confirmation of these thresholds across numerous wine regions and for a wider range of varieties. At the other end of the formula, the calculation for growing degree-days used in
viticulture Viticulture (, "vine-growing"), viniculture (, "wine-growing"), or winegrowing is the cultivation and harvesting of grapes. It is a branch of the science of horticulture. While the native territory of ''Vitis vinifera'', the common grape vine ...
and
wine production Wine is an alcoholic drink made from fermented fruit. Yeast consumes the sugar in the fruit and converts it to ethanol and carbon dioxide, releasing heat in the process. Wine is most often made from grapes, and the term "wine" generally refers ...
does not normally use an upper cut-off. Conceptually an upper cut-off would be applied if the plant system stopped being photosynthetically active at some point due to heat stress from high temperatures. While this may be proven for some crops, there is not a universal number for an upper threshold for grapes so the majority of the published data for comparison purposes in
viticulture Viticulture (, "vine-growing"), viniculture (, "wine-growing"), or winegrowing is the cultivation and harvesting of grapes. It is a branch of the science of horticulture. While the native territory of ''Vitis vinifera'', the common grape vine ...
and
wine production Wine is an alcoholic drink made from fermented fruit. Yeast consumes the sugar in the fruit and converts it to ethanol and carbon dioxide, releasing heat in the process. Wine is most often made from grapes, and the term "wine" generally refers ...
does not limit maximum temperatures. This issue is problematic because many weather stations today have integrated the corn growing degree-day method in their software. The corn growing degree-day method uses both a base temperature adjustment and an upper threshold, neither of which are common in
viticulture Viticulture (, "vine-growing"), viniculture (, "wine-growing"), or winegrowing is the cultivation and harvesting of grapes. It is a branch of the science of horticulture. While the native territory of ''Vitis vinifera'', the common grape vine ...
and wine production use, and can confound any comparison with published data using the simple average method. Furthermore, more complex climate indices have been introduced to address perceived shortcomings in the Winkler index including the Huglin Index, the Biologically Effective Degree-Day Index, and the Multicriteria Climatic Classification system (Geoviticulture MCC). These indices attempt to account for day length and solar, frost, and drought variability that can be found in different locations. Each have been used in various research settings, but have some limitations to the general user in that some variables needed to calculate the indices are not readily available from all weather/climate stations and/or to the general public. Overall each of these issues needs to be carefully considered when comparing growing degree-day values from published data in magazines, books, scientific articles, and even from growers in the same region.


See also

*
Growing degree-day Growing degree days (GDD), also called growing degree units (GDUs), are a heuristic tool in phenology. GDD are a measure of heat accumulation used by horticulturists, gardeners, and farmers to predict plant and animal development rates such as the ...


Notes


Further reading

* * * * * * * Jones, G.V., Reid, R., and A. Vilks (2012). Climate, Grapes, and Wine: Structure and Suitability in a Variable and Changing Climate pp 109–133 in The Geography of Wine: Regions, Terroir, and Techniques, edited by P. Dougherty. Springer Press, 255 pp. {{portal bar, Drink Viticulture Agricultural terminology California wine Climate of California Climate and weather classification systems Horticulture University of California, Davis