Oakville Climate
The climate of Oakville is the unique result of a number of natural weather influences. The cool northern reaches of the San Francisco Bay and the warmer inland air of California’s central San Joaquin Valley combine to provide the temperate Mediterranean climate in Oakville. The gentle tug of war between these cooling and heating influences creates the daily weather patterns that make Oakville an ideal place to grow grapes.
In general terms, and as American wine regions are classified, the climate of Oakville is considered moderately warm. The University of California at Davis developed a system of classifying growing regions of the state, from the coolest (Region I) to the hottest (Region V). The Napa Valley becomes warmer as it extends north from Carneros and the San Pablo branch of San Francisco Bay, with the Mayacamas range to the west protecting the valley from incursions by cold Pacific Ocean air. Driving north through Carneros, then Napa and Yountville, you are in what is considered a Region II growing area. Then, just at the small town of Oakville, Napa Valley transitions from a Region II growing area (similar to Bordeaux) to a Region III growing area (similar to the Rhone).
What makes Oakville’s climate distinctive is its transitional, mid-valley location. Oakville is just far enough south to receive regular morning fog from the San Pablo bay. This slows the warming early in the day. But after the fog blows off in mid-morning, Oakville receives the full benefit of the afternoon Napa Valley sun. In the afternoon, when temperatures begin to peak, the bay blows cool breezes north to Oakville to begin again the cooling cycle of night that preserves the color and acidity of wine grapes. The result is that during most of the growing season, Oakville is about one degree cooler than Rutherford, three degrees cooler than St. Helena, and eight degrees cooler than St. Helena. Average low temperatures in Oakville are generally in the low 50s in March, with highs in the mid-80s during July and August. Daytime highs often reach the mid-90s in mid-summer before the sun sets behind the Mayacamas Mountains.
Rainfall also increases from the south to the north. Oakville usually receives around 35 inches of rain per year, almost all of which falls in winter and early spring. Each spring, the rains dissipate in March and April, and Napa Valley receives negligible rain through the end of the growing season in October. November brings the beginning of the new rainy season, and heavier rains and occasional flooding in December and January replenish the Napa Valley watershed.