Methodology
The data
TerraClimate (Abatzoglou, Dobrowski, Parks and Hegewisch, 2018, Scientific Data; version 1.1, University of California Merced) is a global record of monthly climate on a grid of about 4 km, from 1950 onward, released under CC0 (public domain). For each region we read one grid cell's monthly mean daily maximum and minimum temperature and total precipitation for every year from 1991 to 2025. The cell is the one nearest a point chosen in the vineyards; where that point falls on water we take the nearest land cell (none of the current regions needed it). The site checks every week for a newly published year.
Wikipedia links, grape berry colour and parentage come from Wikidata (CC0). We do not reuse Wikidata's short descriptions, which for many grapes are wrong (several French grapes are labelled "Italian grape variety"). Photos come from Wikimedia Commons, chosen by hand from files whose licence allows reuse (public domain, CC0, CC BY, CC BY-SA), stored on this site and credited where they appear. The world map and the outline of France are drawn from Natural Earth (public domain). Headings use the typefaces Bodoni Moda and Josefin Sans, both under the SIL Open Font License 1.1 and served from this site (licence texts: Bodoni Moda, Josefin Sans). The grape-cloud map of France is our own drawing. The main grapes listed for each region are general wine knowledge, not a dataset.
Growing degree days
For each month of the growing season: GDD = max(0, (mean daily high + mean daily low) ÷ 2 − 10°C) × days in the month, summed over the season. This is the monthly-average form that Amerine and Winkler used. The growing season is April–October in the northern hemisphere and October–April in the southern, where a vintage is named after the year it is picked, so the 2015 southern vintage is the season from October 2014 to April 2015. In °F, GDD are multiplied by 1.8 (base 50°F).
Winkler regions: I below 1,390 GDD; II 1,390–1,670; III 1,670–1,940; IV 1,940–2,220; V above 2,220 (°C units).
The normal, and what "warmer" means
A region's normal is the average of the 30 growing seasons that began in 1991–2020, computed from the same yearly data as the vintages so the two compare like for like. A vintage is near normal within ±3% of that GDD figure, warmer or cooler from 3% to 8%, and much warmer or much cooler beyond 8%. A typical year-to-year spread (one standard deviation) is 4–8% depending on the region.
Season rain is total precipitation over the same months. Harvest rain is precipitation in the region's usual picking months, shown on each region page (for example September–October in Bordeaux, March–April in the Barossa). Hot months are growing-season months whose mean daily high was 2.0°C or more above that month's normal.
Checking the numbers
Before publishing we compared each region's computed normal with published Winkler classifications. Every one falls in or next to the published class:
| Region | Our 1991–2020 GDD | Our Winkler region | Published classification |
|---|---|---|---|
| Napa Valley | 1,930 | III | II–III (Napa Valley; Oakville about III) |
| Mosel | 1,060 | I | I |
| Barossa Valley | 1,870 | III | III–IV |
| Bordeaux | 1,560 | II | II |
| Champagne | 1,180 | I | I |
| Burgundy (Côte d'Or) | 1,410 | II | I–II |
| Marlborough | 1,240 | I | I |
| Hunter Valley | 2,380 | V | V |
| Willamette Valley | 1,230 | I | I–II |
| Central Otago | 880 | I | I |
| Stellenbosch | 2,160 | IV | III–IV |
| Mendoza (Luján de Cuyo) | 2,090 | IV | III–IV |
| Jerez | 2,610 | V | V |
Central Otago reads at the cool end of Region I. Its vineyards sit on warm, sheltered terraces in country whose ~4 km grid cells also take in cold hillside, so the cell is cooler than the vines. Similar, smaller effects apply wherever vineyards hug a slope or a river. Coastal regions such as Sonoma and Margaret River vary from one end to the other more than any single cell can show.
The 2003 European heatwave. 2003 was the hottest summer on record in western Europe at the time, and the data show it: 15 of 21 European regions here were more than 8% above their normal heat that year, led by Wachau (+26%), Burgundy (Côte d'Or) (+22%), Rheingau (+21%). North American regions were not hot that year (Finger Lakes and Niagara were cooler than normal), which also matches the record.
Known limits
- One cell, not a whole region. Large regions span many microclimates; the figures describe one representative 4 km cell.
- Monthly data. Averages for each month hide single heat spikes, frost nights and hail. A month that was 2°C hotter than normal on average almost certainly had very hot days, but a cool month can still hide a damaging frost.
- Modelled, not measured. TerraClimate combines station-based climate normals with reanalysis data. It tracks year-to-year changes well but can be a degree or so off a specific weather station.
- Weather, not wine. Nothing here is a quality rating.