In short, it's Californian—very Mediterranean. It's a great place to be a grape, and a very good one to be a human.
The Napa Valley is far enough inland to escape most of the fog that lingers along the California coast during summer. Yet, unlike California's Central Valley, it's close enough to the ocean to take advantage of the cooling effect of that fog.
It can get hot in the summer, but usually not too hot. We can expect a week or two during the summer when the temperatures are around 100° F, usually in July or August, which are the hottest months. The southern end of the valley, where the city of Napa is located, is cooler. It's closer to the northern tip of San Francisco Bay, known as San Pablo Bay. Winds come through the Golden Gate, move upward and cool off the southern end of Napa County as far as Yountville. They also come over the hills from the west and Sonoma County's coast.
North of Yountville the valley doglegs to the left (west). The late afternoon wind from the bay doesn't make the turn, so the St. Helena and Calistoga areas tend to be much warmer. During the summer, the temperature upvalley can be ten to twenty degrees (F) warmer than in the Napa area.
In the winter, it rains—but not a lot. The county's average annual rainfall for the entire year is less than 24 inches, with over half of that in December, January and February. And it's not unusual to have temperatures in the 70s and 80s (F) around Christmas time.
The western side of the valley—the Mayacamas Mountains—gets more rain, apparent by the redwood and fir forests and numerous streams and waterfalls. The eastern side of the valley—the Vacas Mountains (named for the Vaca family that settled here in 1841)—tends to be more desert-like in many areas, with scrub brush and even cactus.
Average Temperatures by Season
Average Temperatures by Month
City of Napa
Located at 38.30°N 122.28°W.
Height 6m / 19 feet above sea level.
City of St. Helena
Located at 38.50° N 122.46° W.
Height 68m / 223 feet above sea level
Average Rainfall for Napa County
Info from: http://www.napanow.com/climate.html