Winkler index

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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.[1][2]

The system[edit]

The system is based on both the hypothesis and observations that grapevines do not grow if the temperature is below 50 °F (10 °C).[2] 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.

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[1][2] 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.[1][2]

One issue with the original work done by Amerine and Winkler[1] 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[3][4] 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).[5][6]

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[7] 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.[7] It is also functionally similar to growing season average temperatures (simple average of temperatures across the seven month growing season).[3][4][5]


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[8] and a publication[9] on cool climate regions from the International Cool Climate Symposium (
Country Wine Region City GST (°F) GDD (°F units) Winkler Region
Argentina Rio Negro Bariloche 55.6 1194 Region Ia
Chile Lake District Puerto Montt 55.8 1233 Region Ia
Denmark Aalborg 55.8 1233 Region Ia
Washington Puget Sound Port Angeles 56.1 1310 Region Ia
Germany Ruwer Kasel 56.9 1472 Region Ia
Sweden Gothenborg 57.0 1502 Region Ia
England Kent East Malling 57.3 1562 Region Ia
Canada Nova Scotia Kentville 57.4 1579 Region Ia
Michigan Leelanau Peninsula Traverse City 57.9 1695 Region Ia
Australia Tasmania Launceston 58.0 1709 Region Ia
New Zealand Central Otago Queenstown 58.1 1733 Region Ia
Netherlands Maastricht 58.3 1772 Region Ia
France Champagne Reims 58.4 1805 Region Ia
Austria Kremstal Krems 58.5 1821 Region Ia
Poland Lubuskie Zielona Góra 58.6 1849 Region Ia
Switzerland Valais Sion 58.7 1871 Region Ia
England Sussex Eastbourne 58.8 1887 Region Ia
Canada Okanagan Valley Vernon 59.0 1926 Region Ia
Germany Rhine Valley Geisenheim 59.4 2003 Region Ib
New Zealand Marlborough Blenheim 59.7 2075 Region Ib
Canada Niagara Peninsula St. Catharines 60.1 2152 Region Ib
France Burgundy Dijon 60.3 2196 Region Ib
Spain Ribera del Duero Valladolid 60.3 2211 Region Ib
France Alsace Colmar 60.4 2218 Region Ib
Hungary Tokaj Tokaj 60.4 2229 Region Ib
Australia Tasmania Hobart 60.4 2234 Region Ib
Oregon Willamette Valley McMinnville 60.6 2273 Region Ib
Romania Zeletin Bacău 60.7 2295 Region Ib
California Central Coast Santa Maria 60.7 2296 Region Ib
France Loire Valley Nantes 61.0 2355 Region Ib
Germany Baden Freiburg 61.2 2403 Region Ib
France Savoie Chambéry 61.5 2454 Region Ib
Ukraine Crimea Simferopol 61.7 2504 Region II
Australia Coonawarra Coonawarra 61.9 2553 Region II
Spain Rias Baixas Vigo 62.2 2619 Region II
New Zealand Hawke's Bay Napier 62.9 2768 Region II
Australia Adelaide Hills Lenswood 63.2 2817 Region II
Portugal Douro Valley Vila Real 63.4 2861 Region II
Chile Valle de Curicó Curicó 63.4 2864 Region II
Italy Piedmont Torino 63.8 2958 Region II
France Bordeaux Merignac 63.8 2961 Region II
Washington Columbia Valley Prosser 64.0 2993 Region II
Italy Alto Adige Bolzano 64.1 3016 Region III
France Northern Rhône Valence 64.1 3027 Region III
Italy Friuli Udine 64.4 3082 Region III
Italy Umbria Perugia 64.6 3132 Region III
Spain Rioja Logrono 64.8 3167 Region III
California Sonoma Valley Sonoma 64.9 3189 Region III
Bulgaria Thracian Valley Plovdiv 64.9 3192 Region III
Russia Krasnodar Krasnodar Krai 65.0 3219 Region III
Australia Yarra Valley Healesville 65.5 3325 Region III
California Mendocino Ukiah 65.8 3384 Region III
Virginia Monticello Charlottesville 66.1 3442 Region III
Australia Margaret River Margaret River 66.2 3472 Region III
Italy Verona Verona 66.4 3509 Region IV
France Languedoc Béziers 66.7 3577 Region IV
California Napa Valley St Helena 66.8 3601 Region IV
California Northern Sonoma Healdsburg 67.1 3650 Region IV
France Southern Rhône Avignon 67.4 3725 Region IV
South Africa Stellenbosch Nietvoorbij 67.5 3751 Region IV
Australia Barossa Valley Nuriootpa 67.6 3756 Region IV
France Roussillon Perpignan 67.6 3769 Region IV
France Corsica Bastia 67.6 3775 Region IV
Spain Catalunya Reus 68.0 3845 Region IV
Portugal Alentejo Evora 68.1 3874 Region IV
Italy Tuscany Firenze 68.3 3907 Region IV
Portugal Estremadura Lisbon 68.7 3995 Region IV
California Lodi Lodi 68.7 4005 Region V
Japan Yamanashi Kofu 69.3 4140 Region V
Morocco Meknes-Tafilalet Meknes 69.4 4149 Region V
Portugal Madeira Funchal 69.8 4243 Region V
Italy Apulia Brindisi 69.9 4250 Region V
Greece Patras Patras 70.1 4292 Region V
Australia Hunter Valley Cessnock 71.0 4497 Region V
Spain Jerez Jerez de la Frontera 71.4 4575 Region V

Issues and limitations[edit]

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.).[6]

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[6] 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 (GIS).[10] Instead of depicting a region as all one Winkler region (Napa Valley AVA being a Region III for example), spatial data summaries[3] 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.[3] 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. 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,[11] 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 (i.e., hourly, daily, or monthly) 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 (the table above was done using monthly climatological normals). 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.[3][12] 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.[1][2] 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.[13]

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.[14] 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,[15] neither of which are common in viticulture and wine production use, and can confound any comparison with published data using the simple average method.[3]

Furthermore, more complex climate indices have been introduced to address perceived shortcomings in the Winkler index including the Huglin Index,[7] the Biologically Effective Degree-Day Index,[16] and the Multicriteria Climatic Classification system (Geoviticulture MCC).[17] 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,[3] 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[edit]


  1. ^ a b c d e Amerine, M.A.; Winkler, A.J. (1944). "Composition and quality of musts and wines of California grapes". Hilgardia. 15 (6): 493–675. doi:10.3733/hilg.v15n06p493.
  2. ^ a b c d e Winkler, A.J.; et al. (1974). General viticulture. University of California Press. ISBN 978-0520025912.
  3. ^ a b c d e f g Jones, G.V.; et al. (2010). "Spatial analysis of climate in winegrape growing regions in the western United States". American Journal of Enology and Viticulture. 61 (3): 313–326. doi:10.5344/ajev.2010.61.3.313. S2CID 93769404.
  4. ^ a b Hall, A.; Jones, G.V. (2010). "Spatial analysis of climate in winegrape-growing regions in Australia". Australian Journal of Grape and Wine Research. 16 (3): 389–404. doi:10.1111/j.1755-0238.2010.00100.x. ISSN 1755-0238.
  5. ^ a b Anderson, J.D.; Jones, G.V.; Tait, A.; Hall, A.; Trought, M.C.T. (2012). "Analysis of viticulture region climate structure and suitability in New Zealand". OENO One. 46 (3): 149–165. doi:10.20870/oeno-one.2012.46.3.1515. ISSN 2494-1271.
  6. ^ a b c Jones, G.V.; et al. (2012). Climate, Grapes, and Wine: Structure and Suitability in a Variable and Changing Climate, in The geography of wine : regions, terroir and techniques. Netherlands: Springer Press. pp. 109–133. ISBN 9789400704640. OCLC 771916683.
  7. ^ a b c Huglin, P. (1978). "Nouveau Mode d'Évaluation des Possibilités Héliothermiques d'un Milieu Viticole". C.R. Acad. Agr. France. 64: 1117–1126.
  8. ^ Robinson, Jancis; Johnson, Hugh (2013). The World Atlas of Wine. United Kingdom: Mitchell Beazley. ISBN 9781845336899. OCLC 859400304.
  9. ^ Jones, G.V.; Schultz, H.R. (2016). "Climate change and emerging cool climate wine regions". Wine & Viticulture Journal. 31 (6): 51–53.
  10. ^ Daly, C.; Halbleib, M.; Smith, J.I.; Gibson, W.P.; Doggett, M.K.; Taylor, G.H.; Curtis, J.; Pasteris, P.P. (2008). "Physiographically sensitive mapping of climatological temperature and precipitation across the conterminous United States". International Journal of Climatology. 28 (15): 2031–2064. Bibcode:2008IJCli..28.2031D. doi:10.1002/joc.1688. ISSN 1097-0088. S2CID 17681312.
  11. ^ National Weather Service, US Department of Commerce, NOAA, National Weather. "About Climate Normals". Retrieved 2017-01-04.
  12. ^ Battany, M. (2009). "Improving degree-day calculations". Practical Winery Vineyard. May/June: 25–26.
  13. ^ Garcia de Cortázar-Atauri, I.; Brisson, N.; Gaudillere, J.P. (2009). "Performance of several models for predicting budburst date of grapevine (Vitis vinifera L.)". International Journal of Biometeorology. 53 (4): 317–326. Bibcode:2009IJBm...53..317G. doi:10.1007/s00484-009-0217-4. ISSN 0020-7128. PMID 19280231. S2CID 25168485.
  14. ^ Jackson, R.S. (2000). Wine science : principles, practice, perception. San Diego: Academic Press. ISBN 978-0123790620. OCLC 162129379.
  15. ^ "NDAWN Corn Growing Degree Days Information". Retrieved 2017-01-04.
  16. ^ Gladstones, J.S. (1992). Viticulture and Environment. Winetitles. ISBN 9781875130122. OCLC 38326786.
  17. ^ Tonietto, J.; Carbonneau, A. (2004). "A multicriteria climatic classification system for grape-growing regions worldwide". Agricultural and Forest Meteorology. 124 (1–2): 81–97. Bibcode:2004AgFM..124...81T. doi:10.1016/j.agrformet.2003.06.001. S2CID 86709875.

Further reading[edit]