What the FOG: Why Grapes Change Color at Veraison

Every summer, Napa Valley vineyards stage a stunning transformation: grapes begin to change color in a process called veraison (that’s ver-ay-zhun, if you want to impress your friends).

But what the FOG is actually happening?

At veraison, grapes switch from growth mode to ripening mode. Until this point, berries are firm, green, and loaded with organic acids, but not much sugar. Then, as veraison begins, the chemistry inside each berry shifts dramatically:

​🍇 For red varieties: Green chlorophyll breaks down and anthocyanins (natural pigments) accumulate, giving berries their deep purple, red, and blue hues.

​🍇 For white varieties: While they don’t turn red, they lose their bright green tint as chlorophyll fades, and take on a warm, golden glow thanks to carotenoids.

​This color change is more than just a pretty picture—it’s a visible sign that grapes are softening, sugars are rising, and flavors are developing. The vine is investing energy into making berries attractive to birds and animals (nature’s seed dispersers), but in Napa Valley, we’re paying close attention for another reason: harvest is inching closer.

​And, of course, Napa Valley’s famed fog plays a quiet but critical role in this process. Those fog-cooled mornings temper the heat of summer days, slowing sugar accumulation and preserving the natural acidity that gives our wines their vibrancy. You could say the fog helps grapes find their best light—without overexposure.
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So next time you stroll past a vineyard and see berries turning color, know that you’re witnessing a fascinating natural shift—and that Napa’s growers are watching carefully, guided by experience, stewardship, and maybe just a little help from the morning fog.