San Francisco
San Francisco Geography & Climate
Geography
San Francisco occupies the tip of a peninsula between the Pacific Ocean and San Francisco Bay, covering about 121 square kilometres.
The hills
Over forty named hills, of which Twin Peaks reaches 282 metres. They are outcrops of chert, sandstone and serpentinite — scraped-off ocean floor welded onto the continent — and the nineteenth-century street grid was simply laid straight over them, which is why some streets reach gradients of over 30 per cent and a few are stepped rather than paved.
The Golden Gate
The strait connecting bay to ocean is barely a mile and a half wide and around 100 metres deep, cut by a river during ice ages when sea level was lower. It is the only sea-level gap in the entire coastal range for hundreds of kilometres, which is why fog, shipping and the bridge are all concentrated there.
The bay
A drowned river valley, shallow and enormous, fed by the rivers draining the Central Valley. Around a third of its original area has been filled in.
Made ground
The Financial District, the Marina and much of the eastern waterfront stand on fill — rubble, sand and, beneath the Financial District, ships abandoned during the gold rush. That fill liquefies in an earthquake, which is why the Marina burned in 1989 while the bedrock neighbourhoods were fine.
Islands
Alcatraz, a sandstone outcrop with almost no soil, used successively as a fort, a military prison and a federal penitentiary; Angel Island, the largest in the bay, which served as the immigration station where arrivals from Asia were detained, sometimes for months, and where their carved poems survive on the barrack walls; and Treasure Island, entirely artificial, built for a 1939 exposition.
Faults
The San Andreas runs just offshore to the west and the Hayward fault through the East Bay. Both are overdue.
Climate Patterns
San Francisco has one of the most unusual climates of any large city, and almost everyone gets it wrong before arriving.
No summer
The temperature barely moves all year: average highs run from about 14°C in January to 21°C in September, and it is rarely hot and almost never freezes. What changes is fog and rain, not warmth.
The fog
From June to August, the Central Valley inland heats to 40°C, the rising air draws in cool marine air, and that air condenses as it crosses the cold California Current offshore. The result pours through the Golden Gate — the only gap in the coastal hills — as a thick fog that can drop the temperature ten degrees in twenty minutes. Locals have named it Karl. The western districts sit in it for weeks while the Mission, a few miles east, is sunny.
Microclimates
The city has genuinely different weather from block to block. Twin Peaks divides sun from fog, and a temperature difference of eight degrees across three miles is normal.
Rain
Around 600 millimetres, almost all between November and March, in a few substantial atmospheric-river storms rather than steady drizzle. Summers are essentially rainless.
What to wear
Layers, always, including in August. The standard local kit is a t-shirt with a jacket carried, because you will cross from sun into fog within a single walk and back again. Shorts are an out-of-town marker for good reason.
Wildfire smoke
Late summer and autumn can bring smoke from fires elsewhere in California, occasionally severe.