Summer and the Blessing of Fog

In summer, the rains that brought budbreak and flowering dry up. The skies are clear and blue, and temperatures rise. And rise. And rise. The growing grapes do not appreciate the heat, especially the grapes that will become red wines. But the Napa Valley, like all of coastal northern California, is blessed by a climate perfect for growing grapes. This is not a contradiction. The difference is in the blessing of fog.

When we discuss fog and climate, we must first talk about the world’s oceans. They cover more of the Earth than land does, their depths are deeper than the Himalayas are high, and they dictate the world's climate. Oceanography, the scientific study of the seas, identifies a “gyre” as large system of rotating ocean currents, usually driven by the winds. These shape the climates of the world, and when we talk about wine-growing regions, the influence of the oceans is paramount. These immense circular wind systems move ocean water from cold to warm areas, and back again to the cold. In the North Pacific, the Gyre brings Arctic water south along the coast of North America; at the Equator the waters turn westward through the warm equatorial tropics. The North Pacific Gyre then brings its waters north along the Asian coast, then east again to the Arctic where the waters cool and the cycle is renewed. This same pattern is repeated in the North Atlantic.

Gyres exist because of the Coriolis Effect: the earth’s rotation causes freely-moving objects, like airplanes and wind currents, to veer toward the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The South Pacific, South Atlantic, and Indian Oceans also have their Gyres, but they move counter-clockwise. The Mediterranean Sea, being almost landlocked, is a special case, with its currents dependent on salinity rather than the wind.

The great wine-making regions of the Earth are generally found along the western coastlines of Northern America, Europe, Africa, and South America. Here, the Gyres sweep colder waters along the coastlines, which moderate the harsh summer heat.

In Northern California in particular, the North Pacific Gyre confers the blessings of fog. Warm air from the interior, meeting the cold water at the coast, generates great billows of fog; these fogs push their way inland, climbing the slopes of the Coast Range and sliding through gaps in the hills, cooling the vineyards and slowing the ripening of the grapes. Wine grapes tend to be at their peak when they have a long hang-time (that is, the time the fruit is on the vine) which promotes a natural balance in the flavors of the juice from those grapes.

The Gyre, in concert with other forces, also acts to lessen the amount of rainfall along the Northern California vineyards. Just as wine grapes grow most successfully in difficult soils with little nutrition but excellent drainage, so they prosper with limited rainfall. California’s climate reliably produces summer droughts every year. The grape vines growing on the slopes, amid the difficult terrains, covered in cold fogs and given minimal rains, tend to produce the most premium wines.

In contrast, grapes grown in the inland valleys receive reliable heat during the growing season, and are usually well irrigated. But the wines they produce are not the premium wines desired by wine connoisseurs; the wines tend to be weak, light in taste and often too sweet.

It may seem odd that the harder it is for wine grapes to mature, the greater the wines they produce, but we know that this is true of more things in life than just wine grapes. The easy life, for grapes as for people, does not guarantee strong results.

Napa Valley's hills have turned brown, pinstriped by the rows of green vines, interrupted by copses of dark green trees. The cool, foggy mornings give way to hot, dry afternoons, and the grapes flourish. Harvest, and the crush, approach.