Fall days in San Francisco are pretty much the best weather days in existence. A perfect fall day in San Francisco is sublime in a way that I’m not yet eloquent enough to put into words.
Yesterday was the Folsom Street Fair, this next weekend is the Castro Street Fair, and all throughout the city, people are basking in the glorious rays of that golden, glowing orb that is so often concealed behind a gray layer of fog. Yes, Karl the Fog, I’m looking at you!
Here’s a quick overview from the NASA Earth Observatory that explains the why San Francisco fog is so strong (and why our fall weather is so amazingly gorgeous):
Intrusions by the marine layer—and all of the accompanying fog and clouds—are routine in San Francisco during the summer. The intrusions are caused by westerly breezes that push cold air inland to replace the warm air rising off of California’s Central Valley. As it did on the day this image was taken, the marine layer often completely envelops the Golden Gate Bridge in a thick cloak of fog and clouds.
In San Francisco, fog is most common during the summer due to a combination of environmental conditions that cause wind patterns and ocean currents in the North Pacific to play off one another. One of the first steps involves the northward migration of the Pacific High, a high-pressure weather pattern that strengthens and moves toward the coast in the spring and summer. The high causes wind patterns to shift, which in turn affects the California Current, a major ocean current that flows from British Columbia south along the Pacific coast.
During the summer, the strengthening of the high pushes the California Current’s surface waters away from the shore. As this happens, deeper, colder water rises up to replace it, a process called upwelling. Upwelling causes water temperatures in the Pacific to be frigid, but biologically productive. When sea breezes blow over this cold water, water vapor is forced to condense out of the air, forming advection fog. A different type of fog—tule fog—forms in San Francisco during the winter due to a separate set of meteorological conditions.