For most of August, SF was socked in by greater-than-usual amounts of fog coursing down from neighboring hills, through the Strait of the Golden Gate, in from the Pacific Ocean.
Cool, stable air masses trapped beneath warm ones form fog, an accumulation of water droplets or ice crystals suspended in the air around the Earth’s surface. Unlike other clouds, fog lies low, its moisture often emanating from nearby bodies of water. When temperature and dew point nearly converge, water vapor condenses into tiny droplets in the air. Fog occurs when humidity approaches 100%, or ambient air temperature drops.
Sea fog forms over the surface of the ocean, due to peculiarities in the salt spray of breaking waves. Except in stormy areas, this type of fog most commonly hugs coastlines, creating high concentrations of airborne salt particles. Condensation on these particles also occurs at the lower humidity in drier air along coastlines. Drizzle happens when fog’s humidity reaches 100% and the minute cloud droplets commingle, when the fog layer lifts and cools or is compressed from above. Over the marine layer, the warmer, drier air mass rules the thickness of the fog. The marine layer and any fog bank within is compressed with high pressure, or expands upward with lower pressure.
The fog of San Francisco Bay is (of course!) its own special sub-type, “advection fog.” Warm, moist air blowing from the central Pacific across the cold waters of the California Current, flowing just off the coast, creates a cool, moist wind, the water lowering the air temperature to the dew point. Coastal Mediterranean climate areas likes ours often have sea fog blowing off the ocean, the warmer Central Valley air lowering the air pressure and forcing the fog inland into the Bay Area. Our city has been nicknamed the “Naturally Air-Conditioned City,” because of the fog rolling in from the ocean in summer.
The fog moves in slowly now, the streets bathed in it, faint light glowing from houses in the mist. Straddling two great bodies of water, with foghorns booming in the Bay and sounding far off in the Pacific Ocean, it’s like we are on the water, at the prow of some gigantic schooner headed out to sea.