The Winkler Index, sometimes known as the Winkler Scale or Winkler Regions, is a technique for classifying the climate of wine growing regions based on heat summation or growing degree-days. In the system, geographical areas are divided into five climate regions based on temperature converted to growing degree-days, and is commonly known as Regions I–V (see below). The system was developed at the University of California, Davis by A. J. Winkler and Maynard Amerine.
The system The system is based on both the hypothesis and observations that grapevines do not grow if the temperature is below 50 °F (10 °C). Each day during the growing season are assigned growing degree-days 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 over 50 °F, or with SI units, degrees Celsius over 10 °C is used.
Calculation of the Winkler Index The Winkler Index, or Growing Degree Days (GDD), measures heat accumulation for vine growth during the growing season (spring and summer). It sums daily average temperatures above a base threshold of 10°C (50°F).
Formula
Where:
: Maximum temperature on day ( d ). : Minimum temperature on day ( d ). : Base temperature, typically 10°C (50°F). If the result is negative, use 0 for that day. ( n ): Number of days in the growing season Northern Hemisphere.
Periods
Northern Hemisphere: April 1 to October 31 (spring and summer). Southern Hemisphere: October 1 to April 30 (spring and summer).
Steps
Collect daily maximum and minimum temperatures. Calculate daily average: . Subtract (50°F) or (10°C). If negative, record 0. Sum daily values over the period to get total GDD
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:
+Winkler index Region/class °F units °C units General ripening capability and wine style Region Ia 1500–2000 850–1111 Only very early ripening varieties achieve high quality, mostly hybrid grape varieties and some V. vinifera. Region Ib 2001–2500 1111–1389 Only early ripening varieties achieve high quality, some hybrid grape varieties but mostly V. vinifera. Region II 2501–3000 1389–1667 Early and mid-season table wine varieties will produce good quality wines. Region III 3001–3500 1668–1944 Favorable for high production of standard to good quality table wines. Region IV 3501–4000 1945–2222 Favorable for high production, but acceptable table wine quality at best. Region V 4001–4900 2223–2700 Typically only suitable for extremely high production, fair quality table wine or table grape varieties destined for early season consumption are grown. The system was originally developed for and is used officially in California 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 grapes 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 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 and other fortified wines.
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, such as Oregon and Washington, along with Canada, South America, Australia, New Zealand, South Africa, and Europe. However, it is less widely used in Europe where the Huglin index 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).
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, Central Otago, and Valais. Region Ia also includes numerous newer regions growing grapes and making wine including southern England, areas in northern Europe, Nova Scotia, and southern areas of Chile and Argentina. Region Ia areas ripen a range of hybrid grapes and some very early ripening V. Vinifera.
Region Ib is slightly warmer, can ripen early varieties such as Chardonnay, Pinot noir, Sauvignon blanc or Riesling with characteristic locations within the Rhine and Mosel valleys, Burgundy and the Loire Valley, or the Willamette Valley in Oregon as good examples. Region II includes cooler locations within areas such as Bordeaux, Coonawarra, and Valle de Curicó in Chile. Warmer areas in these wine regions fall in a Winkler Region III as do much of the Northern Rhône, Rioja, Umbria, and the Margaret River.
Region IV includes portions of the Napa Valley, Stellenbosch, Corsica, Tuscany, and Alentejo where the warmer climates allow for the ripening of later varieties such as Cabernet sauvignon, Sangiovese, and Syrah. The warmest areas are found in Region V and include areas in the central valley of California, inland Australia and wine producing regions in Morocco, Madeira, Apulia, and Jerez.
+Table of wine regions in various countries worldwide. The city represents the location of the weather station that was used to calculate the growing season average temperatures (GST) and growing degree-days for classifying into Winkler Regions. The data represent either the 1981-2010 climatological normals or period of record for that station. Data come from the World Atlas of Wine and a publication on cool climate regions from the International Cool Climate Wine Symposium (ICCWS) CountryWine RegionCityGST (°F)GDD (°F units)Winkler RegionArgentinaRio NegroBariloche55.61194Region IaChileLake DistrictPuerto Montt55.81233Region IaDenmarkAalborg55.81233Region IaWashingtonPuget SoundPort Angeles56.11310Region IaGermanyRuwerKasel56.91472Region IaSwedenGothenborg57.01502Region IaEnglandKentEast Malling57.31562Region IaCanadaNova ScotiaKentville57.41579Region IaMichiganLeelanau PeninsulaTraverse City57.91695Region IaAustraliaTasmaniaLaunceston58.01709Region IaNew ZealandCentral OtagoQueenstown58.11733Region IaNetherlandsMaastricht58.31772Region IaFranceChampagneReims58.41805Region IaAustriaKremstalKrems58.51821Region IaPolandLubuskieZielona Góra58.61849Region IaSwitzerlandValaisSion58.71871Region IaEnglandSussexEastbourne58.81887Region IaCanadaOkanagan ValleyVernon59.01926Region IaGermanyRhine ValleyGeisenheim59.42003Region IbNew ZealandMarlboroughBlenheim59.72075Region IbCanadaNiagara PeninsulaSt. Catharines60.12152Region IbFranceBurgundyDijon60.32196Region IbSpainRibera del DueroValladolid60.32211Region IbFranceAlsaceColmar60.42218Region IbHungaryTokajTokaj60.42229Region IbAustraliaTasmaniaHobart60.42234Region IbOregonWillamette ValleyMcMinnville60.62273Region IbRomaniaZeletinBacău60.72295Region IbCaliforniaCentral CoastSanta Maria60.72296Region IbFranceLoire ValleyNantes61.02355Region IbGermanyBadenFreiburg61.22403Region IbFranceSavoieChambéry61.52454Region IbUkraineCrimeaSimferopol61.72504Region IIAustraliaCoonawarraCoonawarra61.92553Region IISpainRias BaixasVigo62.22619Region IINew ZealandHawke's BayNapier62.92768Region IIAustraliaAdelaide HillsLenswood63.22817Region IIPortugalDouro ValleyVila Real63.42861Region IIChileValle de CuricóCuricó63.42864Region IIItalyPiedmontTorino63.82958Region IIFranceBordeauxMerignac63.82961Region IIWashingtonColumbia ValleyProsser64.02993Region IIItalyAlto AdigeBolzano64.13016Region IIIFranceNorthern RhôneValence64.13027Region IIIItalyFriuliUdine64.43082Region IIIItalyUmbriaPerugia64.63132Region IIISpainRiojaLogrono64.83167Region IIICaliforniaSonoma ValleySonoma64.93189Region IIIBulgariaThracian ValleyPlovdiv64.93192Region IIIRussiaKrasnodarKrasnodar Krai65.03219Region IIIAustraliaYarra ValleyHealesville65.53325Region IIICaliforniaMendocinoUkiah65.83384Region IIIVirginiaMonticelloCharlottesville66.13442Region IIIAustraliaMargaret RiverMargaret River66.23472Region IIIItalyVeronaVerona66.43509Region IVFranceLanguedocBéziers66.73577Region IVCaliforniaNapa ValleySt Helena66.83601Region IVCaliforniaNorthern SonomaHealdsburg67.13650Region IVFranceSouthern RhôneAvignon67.43725Region IVSouth AfricaStellenboschNietvoorbij67.53751Region IVAustraliaBarossa ValleyNuriootpa67.63756Region IVFranceRoussillonPerpignan67.63769Region IVFranceCorsicaBastia67.63775Region IVSpainCatalunyaReus68.03845Region IVPortugalAlentejoEvora68.13874Region IVItalyTuscanyFirenze68.33907Region IVPortugalEstremaduraLisbon68.73995Region IVCaliforniaLodiLodi68.74005Region VJapanYamanashiKofu69.34140Region VMoroccoMeknes-TafilaletMeknes69.44149Region VPortugalMadeiraFunchal69.84243Region VItalyApuliaBrindisi69.94250Region VGreecePatrasPatras70.14292Region VAustraliaHunter ValleyCessnock71.04497Region VSpainJerezJerez de la Frontera71.44575Region V
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) are excluded; among them sun exposure, latitude, precipitation, 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.