Measuring Wine Climates: Winkler, Huglin, Growing Season Temperature and Beyond

Why one climate needs several numbers

Köppen–Geiger classes (Peel et al., 2007) sort the world by monthly temperature and rainfall through the whole year. They say whether a place is Mediterranean, oceanic or humid subtropical, which is where our article on the three major wine climates starts. They do not say how fast grapes ripen, so viticulture also uses indices built for one growing season.

Each index compresses the same daily temperature record in a different way. Hall and Jones (2010) calculated four indices for Australian geographical indications and found that the same region can fall into different classes depending on which index is used. The sections below take the main scales in turn.

The Winkler index: degree-days above 10 °C

The Winkler scale was published in 1944 by Amerine and Winkler of the University of California, Davis, to divide California into heat regions. Its degree-days are the sum, from 1 April to 31 October (northern hemisphere), of the daily mean temperature above 10 °C (50 °F); a day at or below 10 °C adds nothing.

The five regions are defined in degree-days Fahrenheit: Region I up to 2500, II 2501–3000, III 3001–3500, IV 3501–4000 and V above 4000. Hall and Jones give the same boundaries in degrees Celsius as 1389, 1667, 1944 and 2222.

The strength of the scale is its simplicity: it needs only a daily mean. That is also its limit. Averaging day and night means a hot afternoon followed by a cold night can score like a mild day with a mild night, and the scale knows nothing about rainfall or day length.

The Huglin index: afternoon heat and day length

The Huglin heliothermal index was proposed by Huglin in 1978 and later defined in an OIV resolution (2012). It sums, from 1 April to 30 September in the northern hemisphere (1 October to 31 March in the south), half of the sum of the daily mean and the daily maximum, each taken above 10 °C, and multiplies the total by a latitude coefficient k: 1.02 at 40–42 degrees, 1.03 at 42.1–44, 1.04 at 44.1–46, 1.05 at 46.1–48 and 1.06 at 48.1–50. The resolution gives no coefficient outside 40–50 degrees.

Because it uses the daily maximum, it rises faster than Winkler in sunny places with hot afternoons. Tonietto and Carbonneau (2004) grade it from very cool (up to 1500) through cool (1500–1800), temperate (1800–2100), temperate warm (2100–2400) and warm (2400–3000) to very warm (above 3000). The German weather service DWD names 1700 to 1800 for Riesling and Pinot noir. The index is silent about nights and rainfall.

Growing season average temperature, with six regions

Growing season average temperature (GST) is the mean temperature from April to October in the northern hemisphere (October to April in the south). Jones groups it as cool (13–15 °C), intermediate (15–17 °C), warm (17–19 °C) and hot (19–24 °C), and puts the range for high-quality wine at about 13–21 °C.

Values from Jones’s tables show the spread: Mosel 13.0 °C, Alsace 13.1, Champagne 14.5, Burgundy (Côte) 15.3, Margaret River 18.6 and Barossa 19.9 °C. They depend on the years averaged, and the table’s reference period should be checked before the numbers are compared with newer data. A seasonal mean is easy to read, but it hides peaks of heat and cold nights.

Cool nights, water and the multicriteria system

Tonietto and Carbonneau (2004) argued that heat alone is not enough and combined three indices in a multicriteria system (MCC): the Huglin index, a Cool Night Index (CI) and a Dryness Index (DI). They applied it to 97 regions in 29 countries.

CI is the mean minimum temperature of September (March in the southern hemisphere). Its classes are very cool nights up to 12 °C, cool nights above 12 up to 14 °C, temperate nights above 14 up to 18 °C and warm nights above 18 °C. DI is a potential soil water balance after Riou, from very dry to humid.

Diurnal range, the daily maximum minus the minimum, is not an index of its own in this system; CI captures the night side of it. The authors link cool nights to colour and aroma in red varieties.

What the numbers miss, and how growers use them

A regional number hides places inside the region. A temperature-based zoning of Bordeaux (Bois et al., 2018) found up to 20 days’ difference in the date grapes reach the same ripeness between sites. DWD notes that slopes facing south, south-west and south-east are about 1.5–2 K warmer than flat ground, which adds 150 to 300 degree-days between April and September. None of the scales above includes soil, wind, frost risk or how the vines are managed.

Growers use the indices as a first filter: to judge which varieties suit a site (Jones gives about 14.0–16.0 °C for Pinot noir and 16.5–19.5 °C for Cabernet Sauvignon), to compare vintages, and to follow warming (Jones and colleagues, 2005, found about 1.3 °C of warming over 50 years across 27 premium regions). A number can start a decision, but the vineyard should be walked before it ends one.

Exercise 1: telling two climates apart

[Question]
Two regions have the same Winkler total. One has hot afternoons and cold nights; the other has mild days and warm nights. Which of the Winkler, Huglin and Cool Night indices could tell them apart, and why?

[Model answer]
Winkler uses only the daily mean, so both regions can score the same. Huglin adds the daily maximum, so the region with hot afternoons scores higher. The Cool Night Index uses the September minimum, so it separates cold nights from warm nights.

[Mark scheme (6 marks)]
• States that Winkler uses only the daily mean (2 marks)
• Explains that Huglin adds the daily maximum (2 marks)
• Explains that the Cool Night Index looks at night temperature (2 marks)

Exercise 2: a slope inside a region

[Question]
A vineyard faces south on a slope in a region whose Huglin value is about 1700. Why might the vineyard’s own value be higher, and what does DWD give for slopes in Germany?

[Model answer]
A regional value is an average, and sites inside a region differ; in Bordeaux ripeness dates differ by up to 20 days. DWD gives slopes facing south, south-west and south-east about 1.5–2 K more than flat ground, or 150 to 300 more degree-days between April and September, so the slope can sit above the regional value.

[Mark scheme (6 marks)]
• Explains that a regional value is an average that hides sites (2 marks)
• Gives the DWD figures of 1.5–2 K and 150–300 degree-days (2 marks)
• Concludes that the slope may exceed the regional value (2 marks)