It sounds a bit like a stomach upset, but thankfully it’s not! Diurnal range is all about temperature. It is the difference between the daytime and nighttime temperature in any particular location; the gap between the hottest and coolest temperature in a 24-hour period. Think of two extremes – the first, the Sahara Desert where the temperature can climb to a scorching 40 degrees in the daytime but plummet to zero at night, so a range of 40. At the other end a balmy, humid day in Singapore where the daytime peak is 32 degrees whilst at night it never drops below a sultry 28, so a diurnal range of just 4 degrees. Now, it goes without saying that neither the Sahara Desert nor Singapore is a good climate for grape growing (but that’s another blog entirely), but why does diurnal range matter when it comes to viticulture?
Diurnal temperature variation has major implications for the quality, structure and balance of a wine. A large diurnal range can help grapes to ripen in a more balanced way. Warmer daytime temperatures help to foster sugar development, but cool nights can help to preserve aromas, freshness and acidity. If it’s hot and dry during the growing season the sugar level in the grapes build up. But cold nights help a vine to recover from the heat of the day so that the berries will retain balancing acidity. Photosynthesis finishes with the start of darkness, but, if the night-time temperature does not fall below a certain level, respiration and flavour and tannin synthesis will continue, resulting in more rapid phenolic ripening of the fruit at lower sugar levels creating an unbalanced wine.
Which areas tend to have cooler night-time conditions?
- Those that are more inland and further from oceans like Douro Valley or much of Central Europe including Austria, Bulgaria or Hungary.
- Regions which have high elevations and mountainous terroir like Uco Valley in Mendoza, Argentina; Elqui Valley in Northern Chile; and Valais in Switzerland. The Central Otago area in the south of New Zealand’s South Island has one of the most dramatic diurnal fluctuations of any wine region in the world. Typically it can be 36°C at 3-5 pm and 13-14°C at midnight, and even cooler at 5am.
So if you like wines that have good natural acidity which gives ‘freshness’, now you know which regions to look for!