Follow that fog! Thus, the Sonoma Coast AVA eventually became a vast area stretching 80 miles north of the Mendocino border and south to the Marin County line. The boundaries of the AVA were essentially defined by the eastward travels of the Pacific fog. Because the fog finds an inland entrance at the Petaluma River "Petaluma Gap," the Sonoma Coast region eventually encompassed the true Sonoma Coast—the one overlooking the Pacific Ocean—and an acreage forty miles inland from the coast.
This explains the inclusion of some parts of the warmer AVAs inland Sonoma Valley, Chalk Hill, and Sonoma Mountain; stretches of the cooler Russian River, Green Valley, and Carneros AVAs; as well as the entire Fort Ross-Seaview AVA.
These areas share a similarly cool and windy, fog-drenched climate, perfect for growing excellent Chardonnay, Pinot Noir and Syrah.