Much of the southeastern part of the county is relatively flat farmland near sea level. The coast range of mountains, east of the Russian River, provides a barrier that protects Sonoma County from the very hot weather of the central valley of California during the summer months. At the same time low elevations within the County receive enough sunshine during the summer without any import of hot air from the interior. The nearby Pacific Ocean, however, provides a source of cool, moist air during the summer, and the steady inflow of marine air holds temperatures at low levels over that part of the County through which it moves. As a result, the warmest area tends to be the Russian River valley near the north end of the county, where mountains deflect the marine air and diminish the cooling effect it provides elsewhere.
Rainfall tends to increase with elevation and also to increase toward the north.
These are the principal factors that influence the distribution of precipitation within the County. The temperature pattern within the County is profoundly influenced by the temperature of the sea-water immediately off the coast. Because of these water temperatures, air temperatures over the land remain very cool during the summer, particularly during the night hours, and the warmest part of the year is found in late summer or in the fall. This pattern is most pronounced near the coast. The greatest temperature variation across the County is found during the summer, a period when sharp contrasts may be found between the marine dominated coastal area and interior areas that are relatively continental in character.