Expect a moderate climate in San Francisco, where cool summers and mild winters almost blend into one. Temperatures in the city seldom rise above 80 F/21 C or fall below 40 F/4 C. Morning and evening fog is common during summer months but rarely remains through the day. (Little or no rain falls June-September.) Winter average maximum temperatures are in the high 50s F/10 C and only drop to the upper 40s F/4 C at night. Rain showers can be common November-March.
The weather in San Francisco literally varies from neighborhood to neighborhood—it may be sunny and pleasant in the Richmond district and foggy and cool in the Sunset district. No season is really out of the question for a visit, though September and October are the warmest and driest.
Nearby Marin County, the East Bay and the Wine Country enjoy similarly moderate weather, although their location, sheltered from the cool ocean winds, keeps them warmer and sunnier than San Francisco (it's also cooler at night). Expect temperatures to reach 85 F/29 C in some Marin County communities. The summer-morning fog or cloud cover usually clears well before noon.