The coldest winter I ever spent was a summer in San Francisco. —Attributed to Mark Twain
. . . And the Fog Will Burn Off By Noon—A Brief Introduction to the Weather of the San Francisco Bay Area
On the “other” coast, they often say, “If you don’t like the weather, wait a few minutes.” In the San Francisco Bay area, the phrase that should be spoken might be “If you don’t like the weather, take a short walk.” In a few hundred yards, the weather can change from gray clouds and drizzle to blue sky. Travel a little farther, and the weather requiring a sweater and parka now requires only shorts and a T-shirt.
Throughout the spring, the Pacific High increases in strength and moves closer to the coast. The combination of increased northwest wind stress and Coriolis force causes the southeastward-flowing California Current to turn to the right, away from shore. The water that moves offshore is replaced by cold, nutrient-rich water that is upwelled near the coast from intermediate water depths. The upwelled water makes the surface water temperature colder in June and July than it is during the winter.
This cold water is part of the “natural air conditioning” for which San Francisco is famous. As summer winds travel over the North Pacific, the air absorbs great quantities of moisture through evaporation. As it approaches the coast, the air is cooled by the sea, and condensation occurs.
Whether the fog is thin and wispy or is so thick and heavy that any- where else it would pass for rain, depends on the temperature of the California Current; and how much moisture is in the air. How far inland the fog travels depends on the temperature in the Central Valley—several days of temperatures over 100° F can draw the fog through the Carquinez Strait to the western edge of the valley.
As the strength of the California Current wanes in August, the fog disappears and “summer” comes to San Francisco from August to October, the three hottest months of the year.