San Francisco Weather
Few American cities have such mild yet varied weather as San Francisco. The hilly topography and small area, with water on three sides, combine with the unique California coastal climate to produce extremely varied microclimates here, often just blocks apart.
The summer is characterized by heavy fog blown in from the Pacific, clearing usually by the afternoons, except on the west side of the city, where the fog may not clear completely. Fog is the reason that air conditioning is never seen. The east side (SF Bay) of the city is the warmest with the most sun.
Rain, only about 20 inches per year, falls mostly in the winter though March, and temperatures rarely fall below 40° at night. Spring and fall are glorious and sunny, with the warmest temperatures in September and October, but rarely exceeding 80°.
Source: Jan Null, Golden Gate Weather Services. See ggweather.com