SFO has included that in their ATIS since the Wright Bros started flying.
The gap has been of varying importance over the years for airline
departures and emergency procedures. Because of the unique topography at
the airport, the weather at the gap, when obscured, with the airport being
clear, is considered a significant local condition.

David Ziemba wrote:

During the summer months, during which the stratus sits off the
Northern California coast threatening to move inland, the KSFO ATIS
frequently contains a comment that reads "FG IN GAP W". Via empirical
observation, I'm assuming that this means that there's fog in the gap
between the hills due west of the airport, south of Mt. San Bruno and
north of the coast range.

I wonder if anyone knows or has a guess regarding why this is mentioned
in the KSFO ATIS. I have two guesses:

First, on the CUIT2 departure, there is a "NOTE: Mt. San Bruno weather
information available on 118.05," and "NOTE: For use by Runways 28L/R
departures when weather conditions permit. Jets 2000' ceiling and three
miles prevailing visibility with five miles to the west and northwest."
This would indicate to me that the CUIT2 may be ill-advised when that
comment exists in the ATIS.

Or, I wonder if this is mentioned just to indicate that VFR traffic
might have some trouble transiting the class B, especially at low
altitudes.

Does anyone know or have other guesses?

- david z.