"The Golden Gate is the only complete breach in the Coast Range, which borders the Paciﬁc for most of California’s length. As a result, the Bay Region is the meeting place of continental and oceanic air masses. Through the funnel of the Golden Gate and San Francisco Bay, the immense aerial forces of sea and land wage a continual war, and the tide of battle often ﬂows back and forth with regularity.
Along most of the Coast Range, the sea air and its fog reach the heads of the canyons and are stopped by the higher ridges from penetrating farther inland. But at the Golden Gate, the only sea-level breach in the mountains, the wind moves through the range, bringing with it the masses of condensed moisture. At the maximum, an estimated million tons of water an hour ﬂoat through the Gate as vapor and fog."
Geologic Map of California from the USGS, with the Coast Range shown in light green, and the break at the Gate in the black square
As grumpy as this summer fog can make me, I can't deny how amazing it is to stand in the Mission and watch the fog pour over Twin Peaks. It's the air and the atmosphere made visible in this incredible, tactile way. A million tons. That's amazing.
Get the short story on San Francisco Summer Fog from Bay Nature Magazine "Cutting Through the Fog: Demystifying the Summer Spectacle" (where the above quote came from) or the long story in this book from the same author.