A popular quote incorrectly attributed to Mark Twain is "The coldest winter I ever spent was a summer in San Francisco". San Francisco's climate is characteristic of the cool-summer Mediterranean climate of California’s coast, generally characterized by moist mild winters and dry summers.
Since it is surrounded on three sides by water, San Francisco's weather is strongly influenced by the cool currents of the Pacific Ocean, which moderate temperature swings and produce a remarkably mild year-round climate with little seasonal temperature variation.
Among major U.S. cities, San Francisco has the coldest daily mean, maximum, and minimum temperatures for June, July, and August. During the summer, rising hot air in California's interior valleys creates a low pressure area that draws winds from the North Pacific High through the Golden Gate, which creates the city's characteristic cool winds and fog. The fog is less pronounced in eastern neighborhoods and during the late summer and early fall, which is the warmest time of the year.
Because of its sharp topography and maritime influences, San Francisco exhibits a multitude of distinct microclimates. The high hills in the geographic center of the city are responsible for a 20% variance in annual rainfall between different parts of the city. In the surrounding communities across the bay to the east it is generally warmer, especially further inland.