When the Central Valley of California heats up in the summer time, the waters off the coast of California remain cold. The temperature of the interior may soar to 100 degrees Fahrenheit, while the ocean surface stays in the 60s. These two regions of extreme temperature difference are separated by coastal mountains all the way from the Oregon border to Mexico. There is only one small costal gap; at the Golden Gate between San Francisco and the Marin Headlands.

When the interior heats up, interior air rises. This leaves a surface vacuum. Nature hates this vacuum, so cold foggy air from the ocean rushes through the Golden Gate and nearby mountain gaps to fill it. Flying Fog spills down hills, boils through passes, and bounces off islands as it hurries eastward.

It's an atmospheric event unlike any other on the planet, and all summer long, SF bathes in rivers of misty, signature fog as scientists fine-tune their understanding of this most unusual of annual spectacles. Not merely hot Central Valley air mixing with cool offshore ocean currents, fog has global connections. The San Francisco Chronicle describes flying fog in detail.