What is Huglin or Heliothermal Index (HI) ?

Grapevine (Vitis vinifera L.) is successfully grown in the northern hemisphere, between the 35° – 51° latitudes, where the temperature sums are above 10°C from 1 April – 30 September.  Taking geographical latitude into account, the HI formula is: where T is the mean air temperature (°C); Tx is the maximum air temperature (°C); d the length of day coefficient ranging from 1.02 to 1.06 (for the latitudes 40°- 50°). The Heliothermal Index (HI) for grape-growing regions ranges from HI-3 [very cool] to HI+3 [very warm] and with double the total annual temperature:

Very warm HI+3 > 3,000 °C  (annual temperature data)

Warm HI+2 >2,400 ≤ 3,000 °C

Temperate warm HI+1 >2,100 ≤ 2,400 °C

Temperate HI-1  >1,800 ≤ 2,100 °C

Cool HI-2 >1,500 ≤ 1,800 °C

Very cool HI-3  ≤ 1,500 °C

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