Winkler index


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. 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. Each day during the growing season are assigned growing degree-days according to the amount that the day's average temperature exceeds this threshold; one degree day per degree Fahrenheit over 50 °F. All days during the growing season are then added up, with the sum of the growing degree-days used to determine the region's classification in the original Winkler index as follows:
Region/class°F units°C unitsGeneral ripening capability and wine style
Region Ia1500–2000850–1111Only very early ripening varieties achieve high quality, mostly hybrid grape varieties and some V. vinifera.
Region Ib2001–25001111–1389Only early ripening varieties achieve high quality, some hybrid grape varieties but mostly V. vinifera.
Region II2501–30001389–1667Early and mid-season table wine varieties will produce good quality wines.
Region III3001–35001668–1944Favorable for high production of standard to good quality table wines.
Region IV3501–40001945–2222Favorable for high production, but acceptable table wine quality at best.
Region V4001–49002223–2700Typically 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. 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 found in cooler climates to bolder, bigger wines often with higher alcohol and lush, darker fruit aromas and flavors that are found in warmer climates. 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 or an upper class limit for Region V. 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 and an upper bound to Region V of 4900 F° units. Furthermore, additional research in other wine regions found that Region I was best divided into a Region Ia and Region Ib.
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.

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.
CountryWine RegionCityGST GDD Winkler Region
ArgentinaRio NegroBariloche55.61194Region Ia
ChileLake DistrictPuerto Montt55.81233Region Ia
DenmarkAalborg55.81233Region Ia
WashingtonPuget SoundPort Angles56.11310Region Ia
GermanyRuwerKasel56.91472Region Ia
SwedenGothenborg57.01502Region Ia
EnglandKentEast Malling57.31562Region Ia
CanadaNova ScotiaKentville57.41579Region Ia
MichiganLeelanau PeninsulaTraverse City57.91695Region Ia
AustraliaTasmaniaLaunceston58.01709Region Ia
New ZealandCentral OtagoQueenstown58.11733Region Ia
NetherlandsMaastricht58.31772Region Ia
FranceChampagneReims58.41805Region Ia
AustriaKremstalKrems58.51821Region Ia
PolandLubuskieZielona Góra58.61849Region Ia
SwitzerlandValaisSion58.71871Region Ia
EnglandSussexEastbourne58.81887Region Ia
CanadaOkanagan ValleyVernon59.01926Region Ia
GermanyRhine ValleyGeisenheim59.42003Region Ib
New ZealandMarlboroughBlenheim59.72075Region Ib
CanadaNiagara PeninsulaSt. Catherines60.12152Region Ib
FranceBurgundyDijon60.32196Region Ib
SpainRibera del DueroValladolid60.32211Region Ib
FranceAlsaceColmar60.42218Region Ib
HungaryTokajTokaj60.42229Region Ib
AustraliaTasmaniaHobart60.42234Region Ib
OregonWillamette ValleyMcMinnville60.62273Region Ib
RomaniaZeletinBacău60.72295Region Ib
CaliforniaCentral CoastSanta Maria60.72296Region Ib
FranceLoire ValleyNantes61.02355Region Ib
GermanyBadenFreiburg61.22403Region Ib
FranceSavoieChambéry61.52454Region Ib
UkraineCrimeaSimferopol61.72504Region II
AustraliaCoonawarraCoonawarra61.92553Region II
SpainRias BaixasVigo62.22619Region II
New ZealandHawke's BayNapier62.92768Region II
AustraliaAdelaide HillsLenswood63.22817Region II
PortugalDouro ValleyVila Real63.42861Region II
ChileValle de CuricóCuricó63.42864Region II
ItalyPiedmontTorino63.82958Region II
FranceBordeauxMerignac63.82961Region II
WashingtonColumbia ValleyProsser64.02993Region II
ItalyAlto AdigeBolzano64.13016Region III
FranceNorthern RhôneValence64.13027Region III
ItalyFriuliUdine64.43082Region III
ItalyUmbriaPerugia64.63132Region III
SpainRiojaLogrono64.83167Region III
CaliforniaSonoma ValleySonoma64.93189Region III
BulgariaThracian ValleyPlovdiv64.93192Region III
RussiaKrasnodarKrasnodar Krai65.03219Region III
AustraliaYarra ValleyHealesville65.53325Region III
CaliforniaMendocinoUkiah65.83384Region III
VirginiaMonticelloCharlottesville66.13442Region III
AustraliaMargaret RiverMargaret River66.23472Region III
ItalyVeronaVerona66.43509Region IV
FranceLanguedocBéziers66.73577Region IV
CaliforniaNapa ValleySt Helena66.83601Region IV
CaliforniaNorthern SonomaHealdsburg67.13650Region IV
FranceSouthern RhôneAvignon67.43725Region IV
South AfricaStellenboschNietvoorbij67.53751Region IV
AustraliaBarossa ValleyNuriootpa67.63756Region IV
FranceRoussillonPerpignan67.63769Region IV
FranceCorsicaBastia67.63775Region IV
SpainCatalunyaReus68.03845Region IV
PortugalAlentejoEvora68.13874Region IV
ItalyTuscanyFirenze68.33907Region IV
PortugalEstremaduraLisbon68.73995Region IV
CaliforniaLodiLodi68.74005Region V
JapanYamanashiKofu69.34140Region V
MoroccoMeknes-TafilaletMeknes69.44149Region V
PortugalMadeiraFunchal69.84243Region V
ItalyApuliaBrindisi69.94250Region V
GreecePatrasPatras70.14292Region V
AustraliaHunter ValleyCessnock71.04497Region V
SpainJerezJerez 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 are excluded; among them sun exposure, latitude, precipitation, soil conditions, and the risk of extreme weather which might damage grapevines. 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. Instead of depicting a region as all one Winkler region, 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.
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, 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. Furthermore, 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 period of 30 years, however if 30 years of data is not available then at the minimum five years should be used. However a five-year period is not directly comparable to a 30-year period. How data are averaged is also very important. While weather stations today can average data to an hour, a minutes or even seconds, historical data used to calculate growing degree-days has been done mostly on daily or monthly averages. Shorter term averaging to minutes, or more commonly hourly, arguably better reflects the true thermal effects on the crops, but will result in growing degree-day values that are lower than both daily and monthly. 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 with no upper temperature cut-off. The first issue is that 50 °F 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, 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 and wine production 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 and wine production 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 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. 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.