At 7:30 AM on Tuesday morning of my visit to the Napa Valley, we drove up to Cain 5 to learn more about Napa’s appellations and climate, followed by a tasting of wines from the mountain AVAs (which will be discussed in a future post).

Among the most memorable moments of the trip, we walked out to the vineyards for an incredible demonstration of what makes the Napa Valley’s climate so special. As we walked, the vista opened up and we could see both down the Valley and up the Valley. Then suddenly we saw it, it being the fog, hanging just below us and streaming up into the Valley with some speed.

The photographs should provide a good visual, but why does the fog roll in and where does it come from? Situated beneath the mountains, the Napa Valley is bounded on both sides with the Mayacamas range to the West and the Vaca range to the East. Moreover, the Napa Valley sits to the west of the hot and dry climate of the San Joaquin Valley (aka Central Valley), where much of California’s bulk wine is produced. The high heat from the Central Valley draws the cool air and moisture from the Pacific Ocean into the San Francisco Bay and then up the Napa Valley, cooling the area as it rolls in. This marine influence has differing results as one works their way up the 30-mile long Valley, finding a difference of as much as 1oF degree/mile, with the Southern end (Carneros) being cooler and experiencing increased rainfall when compared to the Northern end (Calistoga).