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Why Grapes Like Cool Nights

Grapes Like Cool Nights – They May be Going Away

Most of us know the general effects of climate change - increasing average temperatures, changing rainfall patterns, and extreme weather events. But vineyard microclimates face other impacts as well.


The Geoviticulture Multicriteria Climatic Classification System (MCC System) was created to assess the impact of climate change in viticulture zones. Composed of 3 key metrics: the Huglin Heat Sum Index (HI), the Dryness Index (DI) and the Cool Nights Index (CI) the MCC System is a powerful tool for understanding localized climate change impact. In this post we examine the Cool Nights Index, however as this is but one indicator, viticulture zoning must be based on broader data collection.


Why Cool Nights Matter

While average air temperature is important for grapes, night temperatures have an even stronger impact on the eventual quality and character of the wine.


The Cool Nights Index (CI) measures the mean minimum night temperature during the month when ripening typically occurs. September in the Northern Hemisphere and March in the Southern. Zones are classifying them as:

Very cool nights (<12°C)

Cool nights (12°-14°C)

Temperate nights (14°-18°C)

Warm nights (18°C or above)


Cool nights (roughly 53-57 °F) are very favorable for ripening, the grapes’ colors and aromas are preserved. A marked difference between the daytime and nighttime temperature also helps to retain acidity and freshness by slowing down the ripening process. Outstanding wines, especially the most highly regarded white wines, are often found in regions with cool nights.


Warm nights can cause a loss of aromas and color, especially in red wines. Warmth also contributes to rapid ripening that can create high alcohol wines with stewed fruit or jammy flavors. On the other hand, areas with very cool nights have lower ripening potential.


Many factors can influence a vineyard’s night temperature including elevation and wind currents. Vineyards with the same average temperature, can have very different ripening conditions due nighttime cooling patterns.


Nights are Getting Warmer – Evidence from Spain


With climate change, nights are getting warmer. In one of the most detailed longitudinal studies of cool nights, a team from University of Vigo examined temperature changes over a 16-year period (2000-2015). Their study covered 5 Destination of Origin (DO) areas in northwest Spain. These diverse DOs include two major climatic regions (Eurosiberian and Mediterranean) and a transition zone. Elevations range from sea level in Rías Biaxas to 700 meters in Valdeorras.


Rias Baixes, is known for Albariño, a dry and highly acidic white wine. Mencia, from Ribeira Sacra, creates a deeply coloured and highly acidic red wine. Treixadura makes a highly acidic white wine often used in blends, it can be found in Ribeiro.


The study found elevated average temperatures throughout northwest Spain between 2000-2015, increases were particularly pronounced in Ribeira Sacra. Moreover, the Cool Night Index showed an increase in temperatures of around 0.5°C per decade. Rías Baixas, with a CI of 13.89°C in 2000-2015, is now at the upper edge of the CI ‘cool nights’ range. Riberio has a CI of 12.16°C. The other 3 DOs, to include Ribeira Sacra (11.34°C), had CIs at the top range of ‘very cool nights’. Using additional datasets, the team found the trend in increasing nighttime temperatures also took place during period 1950-2000.


So what are the implications? Based on an analysis of a wide range of indices, in addition to CI, there is likely to be a greater variation in the acidity levels of wines in Northwest Spain and higher levels of alcohol. This trend is most noticeable in DOs that have a Mediterranean climate, such as Valdeorras. These trends could result in growers adopting new cultivars, a shift in viticultural practices or even increased cultivation at higher altitudes which currently experience colder weather.


For a deeper understanding of the impact of climate change on Spain’s viticulture zones, I strongly recommend reading the entire article (available in our Research tab).



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