This French term has been adopted by English speaking wine grape growers and simply refers to the onset of ripening. But like many technical terms used in the wine industry its current meaning has taken on a slightly different context than its original usage. While grape clusters are always developing, veraison refers specifically to the time when the grape vine ceases vine growth (development of leaves and lengthening of shoot growth) and directs photosynthesis to ripening its fruit. With red grape varieties it also refers to the onset of color development in the grape skins. In both white and red varieties it signals the rapid accumulation of sugar. During this time naturally occurring acids in the fruit begin to diminish. Seeds mature from soft green to hard and brown.
The wine grape species used to make essentially 100% of the world's wine grape production (Vitis vinifera) originally came from the Caucuses region of Asia and the land surrounding the Mediterranean Sea. It's hot and dry in the late summer, so it's not difficult to accept that the grape vine evolved to adapt to this environmental hazard by directing its energy into fruit and seed development, thus ensuring the next generation of grape vines. V. vinifera's transplantation to the North American East Coast presents the winemaker a big problem - it never stops raining. Thus the cue to stop growing and start ripening fruit is weakened. It's not uncommon to harvest fruit from vines with active shoot growth and green stems. Exacerbating this delay of ripening is that fact that the East Coast experiences a relatively short growing season. In Northern Virginia bud break can happen almost a full month after bud break in the same latitude in California.
The East Coast also presents winemakers another challenge - warm nights. While photosynthesis is responsible for much of what we think of during ripening, it's completely irrelevant to the metabolism of acid (malic acid) inside the berry. And extended periods of warm temperatures favor the rapid decrease in malic aid content. The risk here is that the berries lose too much acid before it achieves other characteristics of ripeness such as sugar development. For varieties that typically go through malolactic fermentation, the winemaker risks wine with low acid and high pH - a recipe for microbial spoilage and flat wine. Viognier is one variety that is thought to not achieve varietal aromas until late in the ripening process (I don't believe this is true however). It's not uncommon to leave Viognier clusters hanging on the vine targeting at least 24° Brix (% sugar). At this point the acid content can dip to ~4 grams/liter with the pH climbing to 4.0, parameters far outside the traditional French model.
These environmental challenges are a worse case scenario. The Mid Atlantic area can be much more mild. For example, the 2008 vintage saw unseasonably cool night time temperatures, dipping into the 50's in August. 2010 was exceptionally dry. And 2014 saw no heat waves at all, maximizing the portion of the day that contributes to photosynthesis. 2017 is turning out to be "normal"- hot and wet. While the popular opinion of wine grape growing tends to dismiss the Continental climate in premium wine grape production, it's actually common in many successful wine regions around the world.
Here in the Mid-Atlantic we address ripeness and climate issues mainly in two ways: 1) planting varieties that tolerate our short, hot, and wet growing season, and 2) manage the grape growing with an intense amount of hands-on canopy management. In the first scenario there are many varieties that respond well to our climate, unfortunately most of these aren't popular or well-known with customers, or simply don't make good wine. Others varieties, like Chardonnay, Albariño, and Merlot already have brand recognition with customers and can make world class wine. Some varieties, however, like Cabernet Franc and Cabernet Sauvignon need more persuasion in the vineyard to coax proper ripeness. Cabernet Franc is notoriously fickle, exhibiting uneven ripeness and susceptibility to incomplete color development in the skin. Cabernet Sauvignon simply requires a longer growing season than the Mid-Atlantic offers most years. In both cases growers adjust the mangement of the canopy to compensate. Many growers reduce the number of clusters per shoot to direct more of the grapevine's ripening potential into fewer clusters. Theoretically this achieves ripeness sooner than if more clusters were left on the vine. While this practice has seen a lot of scrutiny and push-back from growers on the west coast, it does seem to be a legitimate vineyard practice in the Mid-Atlantic.
While harvest decisions can be framed in an overly pedantic manner, it becomes much more obvious in real world scenarios. Often the decision is made by imminent rain in the forecast. Other times disease pressure has set in and harvesting the fruit now is critical. But even without those pressures there is a certain intuitive feel for when a cluster is ripe. The sweetness of the fruit against the acidity, the feel of the skins on the palate, or the mild tannin of mature seeds. With proper variety selection and grape vine management we hope to make harvest decisions in this context, not chasing ripeness where it won't happen.