Chapter 15: California

San Francisco is the leading financial and cultural center of Northern California and the San Francisco Bay Area. The only consolidated city-county in California, it encompasses a land area of about 46.9 square miles (121 km²) on the northern end of the San Francisco Peninsula, giving it a density of about 17,179 people per square mile (6,632 people per km²). It is the most densely settled large city (population greater than 200,000) and the fourth most populous city in the state of California.
 

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 moderates temperature swings and produces 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. The nearby San Andreas and Hayward Faults are responsible for much earthquake activity, although neither physically passes through the city itself. The San Andreas Fault caused the earthquakes in 1906 and 1989. Minor earthquakes occur on a regular basis. The threat of major earthquakes plays a large role in the city's infrastructure development. The city has repeatedly upgraded its building codes, requiring retrofits for older buildings and higher engineering standards for new construction. However, there are still thousands of smaller buildings that remain vulnerable to quake damage.

References:
http://en.wikipedia.org/wiki/San_Francisco