Bioclimatic indices of the ripening period (2) : Thermal conditions at night

The Cool Night Index

The Cool Night Index was created to study nocturnal conditions of grapes ripening according to the different wine-making regions (Tonietto, 1999 ; Tonietto et Carbonneau, 2004). By convention, this index is also called nycthermal index and is equivalent in the North Hemisphere to the minimum temperature of the harvest month which is most of the time September. In the South Hemisphere, the minimum temperature of March is taken into account. There are 4 classes for night conditions as shown below in table 1.

Cool Night Index

Nighttime coolness and Wine characteristics

The nocturnal coolness is an important factor during the grapes ripening period. Vaudour (2003) explains that « this Cool Night Index of Tonietto is based on the relationships between cool temperatures and the biosynthesis of aromatic compounds during the ripening period, within the 30 days before harvest ».

A good synthesis of sugar in berries would usually be associated with wine regions experiencing a minimum temperature below 10°C (50°F) and a wide diurnal temperature range, during the period preceding harvest. Fregoni et Pezzutto (2000) explained that the last 10 days before harvest are crucial for quality and pointed out that « grapevines feel difficulties to ensure the transfer of compounds from photosynthesis, when temperatures stay high at night (like in climates with small thermal variations) ». For all these reasons, « the best organoleptic characters in grapes are obtained under climates with important diurnal thermal differences during the period preceding harvest » (Fregoni et Pezzuto, 2000).

Many other authors showed the relationships between minimum temperatures and/or diurnal contrasts and wine Characteristics. For instance, Winkler et al. (1974) showed that a small difference in temperatures between day and night give less marked colors on berries skin than if differences were greater. Jackson and Lombard (1993) reported that cool nights associated with warm day temperatures, i.e., great daily thermal amplitude, were able to decrease acid levels in grapes compared to cooler daytime and warmer nighttime conditions. More recently, Yamane et al., 2006 and Koshita et al., 2007 showed that « a large temperature range between nighttime and daytime allows a good biosynthesis of anthocyanins and abscisic acids giving grapes their skin color and wine its quality. »

Towards a new approach of the Cool Night Index

The Cool Night Index can be criticized since it is only based on the minimum temperature of one month, the one before harvest, specially if local scales are investigated. It seems more judicious to actually go into deeper details by calculating the amount of time during which grapes are exposed to different ranges of temperatures rather than the value of the monthly minimum temperature. This approach takes better into account the climate conditions within a complex terrain and specially under radiative weather conditions (Bonnardot et al., 2012). Indeed, spatio-temporal fine scale variations such as thermal inversions, common during clear sky nights, would be better considered using this approach. This still has to be further investigated in relation with berries composition.

Please find previous article of the serie here


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. Cyril,

    Many times I have written about this index being too simplistic and inappropriate to truly capture this effect. For example, a mapping of the CI over the west coast produces nearly every region to be a CI4 or the coolest category. While there have not been any other developments to craft an index for this effect, it must include the DTR but also some measure how high the day Tmax is and how low the night Tmin is because all DTRs are not equal in their effect. Also, this time of year many locations in the western US have DTRs that are double or more compared to those in Europe. But do vines really care about temperatures below a give point, or is it the lower daytime temperatures coupled with some period at a given low temperature that is critical. There are many questions here and one would need to do the same research in many regions (climates) with many varieties to craft a true ripening cool night index.


  2. I really like what you guys are up too. This type of clever work and reporting!
    Keep up the superb works guys I’ve added you guys to my own blogroll.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.