A vertical fall of fog flowing down over the crest of a hill (cf. waterfall).

Visitors to San Francisco, if they're fortunate, may get to see this phenomenon in the autumn, as fingers of fog trickle down Twin Peaks into the Castro, "like a slow-motion Niagara.*" It happens in the summer, too, but you won't be able to see it in San Francisco because you'll be shivering in the middle of the fog bank.

Best time to see fogfall: late summer, late afternoon, as the sun provides nice backlighting. Best direction: Look west, silly, that's where the Pacific Ocean is. Best places to see fogfall are actually outside of San Francisco proper, just south or north of the city, and just east of the ridgeline separating the Pacific Ocean from the San Francisco Bay:

Some pictures, to give you an idea (nothing like the real thing, though):

*Harold Gilliam, San Francisco Chronicle, June 12, 2000.