Bioclimatic indices of the ripening period (1): The Huglin index

The amount of sunshine: critical criteria for grapes ripening

There are a few important indices you want to look at closely when it comes to the ripening period for grapes and the Huglin index is definitely one of them. This index, unlike the Winkler index (Growing Degree Days), takes into account the duration of the day thanks to a coefficient named « day length » (Table 1). This coefficient brings into the formula the concept of the potential amount of light the plant receives, grapevine growth being strongly related to hours of sunshine. This corrects possible gaps from the simple formula of the growing degree days (Winkler index).

huglin IndexTable 1. Examples of « day length » coefficients according to latitude positions (from Vaudour, 2003)

How is the Huglin index calculated?

 In the North Hemisphere the Huglin index is:

Huglin index

In the South Hemisphere, this index will be calculated over the period « October 1st – March 31st ». This index was created by Huglin in 1978 after several tests were applied on Winkler and Branas indices with the ampelographic collection of the agronomic station of the French agronomic institute in Colmar, France. In the end, the Huglin index, calculated over a 6 months’ period, was way more correlated with the sugar content of hundreds of varieties (Vaudour, 2003).

Huglin index

Table 2. Correlations between different bioclimatic indices and sugar content in grapes (adapted from Huglin, 1978)

Maximum temperatures, integrated in the Huglin formula, allow to differentiate vineyards with similar mean temperatures, but different viticultural potentials according to daily temperatures range. Tonietto proposed a classification of viticultural climates based on the Huglin index calculation applied to around thirty wine countries. As a result, he obtained 6 different classes characterizing viticultural regions from the coldest to the warmest. Table 3 shows these classes and some examples of viticultural regions. The examples of the viticultural regions in the table are the one observed over the 1961-1990 period when Huglin performed his study.

Huglin indexTable 3. Huglin index classes (from Tonietto, 1999)

While the Winkler index, is widely studied because correlations between this index and the different phenological stages are statistically significant, the Huglin index is interesting to follow between veraison and harvest in order to highlight ripening potentials of grapes. Wine-makers are used to do samples right before harvest to make sure grapes reach the right balance between sugar and acidity. Huglin index maps can help them to manage more efficiently their sampling and schedule grapes picking itinerary. Indeed, Huglin supplied comparisons between values of sugar content, and proposed different values depending on the grape variety. The data on which the statistical study was based are however limited in space, despite the large number of grapevine varieties. Nevertheless, Huglin proposed index values for different grape varieties, to allow a sugar content of 180 – 200 g/l (Table 4).

Huglin index

Table 4. Huglin index values allowing a sugar content of 180-200 g/l (according to Huglin experiment in 1978)


Posted by Cyril Bonnefoy

climatologist, and CEO of TerraClima has been working since 2006 on understanding the relationships between climate and grapevines. He is specialized in assessing the climate variability at fine-scales and has developed an innovative model which allows creating high-resolution maps (10 meters/32 feet) for wineries. His company, TerraClima, provides fine climatic data to help winemakers to minimize climate risks (frost, heat waves) and manage their harvest.


  1. We have a much simpler way of predicting harvest dates. If you count the number of days from flowering to harvest on your old records, you’ll probably find there are remarkably repeatable and recurring numbers that hardly vary for the given wine styles they are grown for. Once flowering occurs in November and December, we have our harvest plan for February through to April, Accurate to a few days which is as much influenced by weekends,public holidays and winery scheduling issues than any other factors. And here’s a tip, harvest is early or late or normal? Look at when the Easter harvest moon falls! Simply put, early Easter, early harvest, late Easter, late harvest. Yes climatic factors influence the date, of course they do, but they correlate with the Easter moon perfectly! Indices are fine but they look at a very narrow data set. It’s far more complicated than that.

  2. The Huglin Index is one of the parameter which is interesting to watch as we get closer to harvest but of course many other parameters should be taken into account : soils, vegetal material, growers practices …Indeed, there is a theorical number of days by varieties between bloom and harvest. The Huglin index is here to help wine makers and growers to refine this prediction. In no case, it will replace grapes samples. It is just an other tool to help wine makers to figure out climate variability inside their blocks.

  3. As you well know, there is a lot more to fruit flavor physiology/harvest than sugar. Skin changes, seed tannins, weather risks are a few not to mention labor, tanks or machine availability. Given that,the historical bloom to harvest is a fairly consistent indicator give or take a number of days. The Huglin may be useful to determine what variety to plant but organoleptics should dictate your picking date if you are not squeezed by other constraints. Cheers and nice work

    1. The Huglin index is a recent index (1978) and therefore not as known as for instance the Winkler index (developed in the early 40’s for the demarcation of wine regions in California).
      However the use of the Huglin index is definitively increasing within the research community due to improving correlation with grapevine phenology or berry components compared to other seasonal climatic indices. Part of our job could be to introduce and wide spray largely this index to wine growers.

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