Green Valley is defined by the fog. Computerized mapping of fog patterns shows that most of the fog originates at the Petaluma Wind Gap in the coastal mountains. Because of its proximity to this gap, Green Valley typically experiences cooler morning temperatures for a longer period of time than do the northern parts of the Russian River. Green Valley is the first place where the fog comes in and the last place where it burns off, making it the coolest, foggiest part of the Russian River Valley.
The fog animation map at right is derived from weather satellite images of fog and low clouds. The animation demonstrates fog movement for the Green Valley-Russian River Valley appellation during a typical 24-hour ripening period prior to harvest.