Two vineyards can ripen grapes to the same sugar level and make completely different wine — one taut and refreshing, the other broad and flat. A large part of that difference is set overnight. The diurnal range, the gap between a site’s daytime high and its night-time low, is one of the most reliable levers in viticulture, and it works almost entirely through acid chemistry.
What happens after dark
During the day the vine photosynthesises, building the sugars that will become alcohol. But the grape is a living organ, and it keeps respiring through the night — burning fuel to stay alive. Once the sun sets and photosynthesis stops, the berry turns to its stored organic acids as an energy source. The key point is that the two acids in a grape are not burned equally: malic acid is far more readily respired than tartaric acid, which is metabolically stable. So night temperature acts as a dial on malic acid specifically, and malate is a large share of a cool-climate grape’s total acidity.
Warm nights speed respiration, and the berry consumes more of its malic acid before harvest — the wine arrives soft, low in acid, short on tension. Cool nights slow respiration to a crawl, and the acid is preserved into the finished wine. This is the mechanism behind the phrase winemakers use for high-diurnal sites: they let a grape reach full ripeness and keep its freshness, two things that usually pull against each other.
Where the swing comes from
A wide diurnal range needs something to strip the day’s heat away quickly after sunset. Three settings do it best:
- Elevation — thinner air holds less heat, so high vineyards cool sharply at night. The Canberra District sits well above 500 metres, and that altitude, not the latitude, is what keeps its wines fresh.
- Continentality — inland sites away from the moderating thermal mass of the ocean swing harder between day and night than coastal ones.
- Clear, dry skies — without cloud cover, daytime heat radiates back out to space overnight, deepening the drop.
Why it shows up in the glass
The acid a cool night protects does more than taste fresh. It holds the wine’s pH down, which sharpens flavour, stabilises colour, and is a major contributor to a wine’s capacity to age — low-pH wines are more microbially stable and evolve more slowly. Cool ripening also favours the aromatic side of the grape: it is no coincidence that the peppery rotundone of cool-climate Shiraz builds in exactly these high-diurnal sites, or that the taut, structured Cabernet of Coonawarra’s terra rossa comes from a genuinely cool corner of South Australia.
The Australian regions prized for freshness are diurnal-range regions. The high-altitude Shiraz of Clonakilla in Canberra and the racy Clare Valley Riesling of Grosset both owe their bright acidity less to how cold the days are than to how cold the nights get.