This Index is based on Maynard Amerine and Albert Winkler's idea of Growing Degree-Days (GDD) — the sum of the daily temperatures above 10 °C (50 °F) during the grapes' growing season. There are several ways that the data necessary for the calculations could be collected for any one spot on Earth, but a standardized version was developed some time ago by Gregory V. Jones and his colleagues. They have published several papers about various regions of the world:
- G. Jones, M. Moriondo, B. Bois, A. Hall, A. Duff (2009) Analysis of the spatial climate structure in viticulture regions worldwide. Bulletin de l’Organisation Internationale de la Vigne et du Vin 82: 507-518.
- Gregory V. Jones, Andrew A. Duff, Andrew Hall, Joseph W. Myers (2010) Spatial analysis of climate in winegrape growing regions in the western United States. American Journal of Enology and Viticulture 61: 313-326.
- Andrew Hall, Gregory V. Jones (2010) Spatial analysis of climate in winegrape‐growing regions in Australia. Australian Journal of Grape and Wine Research 16: 389-404.
- Gregory V. Jones, Fernando Alves (ms.) Spatial analysis of climate in winegrape growing regions in Portugal. Unpublished manuscript.
Of interest to us here is the fact that the above publications each contains a gridded map showing the Winkler Index zones for a different part of the globe. I have reproduced here a version of the maps for the western USA (California, Oregon, Washington, and Idaho) and for Australia.
The map of the western USA is based on 400x400 m cells = 16 ha = 40 acres. The colors refer to the five Winkler Index zones (I-V with increasing heat summation). Most of the northern areas of the map are unsuitable for viticulture because the heat summation is too low (<850 degree-days), while the south-eastern parts have a heat summation that is too high (>2700 degree-days). The Central Valley of California is clearly indicated in red, being the hottest place with grapevines in the western USA.
This map can be compared to the boundaries of the American Viticultural Areas (AVA), which would indicate which grape varieties might be most suitable for each AVA. However, the spatial variability of the GDD values within any one AVA can be large. For example, the best-known AVA, Napa Valley, mostly lies within Region III, but it actually ranges from Region I to the lower part of Region V. The most important cause of this effect throughout the western USA is variation in altitude within the AVAs. Consequently, parts of most AVAs have never been planted with grapes, and probably never will be.
The grid cells of the Australian map are 25 sq.km each, since the map covers the whole continent. Note, however, that very little of Australia is actually suitable for viticulture, unlike North America or Europe, because the heat summation is way too high. Only in the south-eastern part are vines absent because the heat summation is too low.
Most of the suitable vine-land area of Australia already has formal Geographic Indication (GI) viticultural areas, which are outlined in red on the map. Those areas with suitable GDD values but that do not (yet) have vines are, in fact, almost all too dry for viticulture.
For Australia, the biggest influences on the differences in GDD values between GIs are latitude and continentality. For example, note that the GDD values generally decrease from north to south down the east coast, with the coolest climates being on the island of Tasmania. Furthermore, the inland GIs are more likely to have greater index values than are GIs closer to the ocean. However, the latter effect is interrupted by the Great Dividing Range, which runs all the way down the east side of Australia. The upland areas are much cooler than are the coastal areas. Indeed, the northern GI areas are all at high altitude.
As noted above for the western USA, even within each of the GI areas, parts have never had grapes and probably never will have, due to variation in altitude. This is particularly important in those GIs located in the Great Dividing Range.
Both Australia and the western USA have areas within all of the Winkler Index zones. For example, Jones et al. list some comparable Australian GIs and western US AVAs, based on their median GDD values:
- Region I – Southern Tasmania (GI) with Puget Sound (AVA) and numerous Willamette Valley sub-AVAs
- Region II – Coonawarra (GI) with Red Mountain (AVA); Upper Goulburn (GI) with the Sonoma Coast (AVA)
- Region III – Bendigo (GI) with Alexander Valley (AVA) and the Dry Creek Valley (AVA); Margaret River (GI) with the Napa Valley (AVA)
- Region IV – Adelaide Plains (GI) with Lodi (AVA)
- Region V – Swan District (GI) with Madera (AVA).
Finally, it is worth noting (as Jones et al. do) that viticultural areas with similar grape varieties and wine-making reputations do not always have the same GDD values. For example, Burgundy (France), the Willamette Valley (USA), and the Yarra Valley (Australia) each excel with Pinot noir and Chardonnay, but the Yarra Valley (median 1558 degree-days) is significantly warmer than is either Burgundy (1118) or the Willamette Valley (1081). Similarly, Bordeaux (France), the Napa Valley (USA), and Coonawarra (Australia) are often compared based on the quality of their Bordeaux-mix of red grapes, but the Napa Valley (1883) is substantially warmer than is either Bordeaux (1410) or Coonawarra (1457). Intriguingly, Margaret River, a GI with a rapidly rising reputation for producing Australia's best Bordeaux-mix wines, is remarkably similar to the Napa Valley (ie. median 1844 degree-days).
There are a lot of limitations of using Growing Degree-Days to assess areas for their climatic suitability for growing high-quality wine grapes, but nevertheless they can be very informative, especially for indicating unsuitable locations.