Wine and Ratings

If you’ve ever encountered the word “diurnal” it was likely during a visit to a California winery or between sips at a winemaker dinner. And then you probably heard it again at the next place you visited and the one after that and so on. Diurnal means “daily,” and in winegrowing regions “diurnal swing” or “diurnal shift” is the difference between the maximum daily temperature and the minimum nightly temperature in a 24-hour period. It is a valid metric for understanding certain aspects of grape and wine quality, but only—in my view—when used in moderation. Some wine producers, eager to convince you that their vineyards—and not the vineyards of their colleagues in other Golden State counties—are especially blessed, are using it in immoderate ways. “Lake County has diurnal swings that other regions can’t duplicate,” boasts Lake County Winegrowers on its website. The swing can be more than 50 degrees and helps produce more complex flavors and balance in both grapes and wine, according to the article. But it seems that other regions can duplicate Lake County’s diurnal swing. In Paso Robles, 300 miles south, winemakers also cite a difference of 50 degrees or more in a day. Paso Robles Wine Country Alliance claims its diurnal shift is “a greater day-to-night temperature swing than any other appellation in California.” I am a fan of both Paso Robles and Lake County wines, but are claims like these helpful when you’re shopping for wine? Let’s take a deeper look: Wine grapes gradually develop sugar, which is fundamental for alcohol and fruit flavors, in the heat and sunshine of summer days. Natural fruit acid in the grapes moves in the opposite direction. It starts high and slowly decreases until harvest. The highest quality wines are generally made from grapes with good balance, that is, sugar that grows just enough and acid that doesn’t drop too low. Since cool nights slow the sugar development and the acid dissipation, the argument goes that they enable better balance in a region with hot days. I discussed the diurnal debate with Colorado-based geographer Patrick Shabram, an expert on grape-growing conditions, who has worked in numerous U.S. wine regions. He says that diurnal shifts are useful when arguing for an area’s unique terroir, but he laughs when I ask if highs near 100 and lows below freezing—a 70-degree swing—would be better than a swing of 50. The sheer size of the diurnal shift is not a proxy for wine quality. “Sometimes it’s the opposite,” says Shabram. Pinot Noir grapes in the West Sonoma Coast adjacent to the notoriously cool Northern Pacific see a modest 20-degree shift in some parts. “If you’re growing Pinot Noir there, you get a lower daily high that prolongs the growing season a little,” Shabram says, “while a higher minimum temperature allows for some metabolism at night. People argue that that is better than a big swing.” The high quality of Pinot Noir, Chardonnay and other wines from lower-shifting regions in California like Santa Barbara, Santa Lucia Highlands and Carneros also supports this observation. Not to mention the classic expression of Cabernet Sauvignon and Merlot wines from Bordeaux, where the July diurnal shift averages only 20-25 degrees in many vineyards. Wine producers and promoters up and down the state would be smart not to devalue the concept of diurnal shift by beating this drum too loudly and too often. It just adds to the marketing noise. And consumers would be smart to take the producers’ claims of diurnal superiority with a few grains of salt. Diurnal shift is one of many factors that make wine a fascinating and complex subject, but it’s not a magic ingredient that necessarily makes one region’s wine better than another’s. This column originally appeared in the April 2023 issue of Wine Enthusiast magazine. Click here to subscribe today!