This phenomenon is caused by the persistence of the Pacific High and the northwest winds that are constrained by the Coast Range to blow parallel to the coastline. The effects of these winds, the Coriolis Force and resultant sub-surface Ekman Spiral, causes a net transport of surface waters away from the shore. The mornings will typically find the entire city overcast followed by clearing on the warmer bay side, but only partial clearing on the cooler ocean side. Rainfall from May through September is relatively rare, with an aggregate of less than an inch, or only about 5 percent of the yearly average total of approximately 21.5 inches. Off- season rains which do occur are usually the result of weak early or late season occluded fronts, or surges of subtropical moisture from the south that result in brief showers or thundershowers spreading into the area.
Considerable moisture is due to drizzle when the marine layer deepens sufficiently.
The main source region of wintertime fog in San Francisco is the Great Valley. Radiation fog is formed in the moist regions of the Sacramento River Delta and is advected through Suisun and San Pablo Bays and into San Francisco Bay on cool easterly drainage winds. Over 80 percent of San Francisco's seasonal rain falls between November and March, occurring over about 10 days per month. Winter rains on the California coast are primarily due to occluded fronts on a trajectory from the west-northwest, and an occasional cold front from the Gulf of Alaska. These systems are driven southward during the winter as the Pacific High drifts south and westerlies and Polar jetstream dip south of 40 N. Winter thunderstorms occur on the average only twice per season in cold unstable post-frontal airmasses.