For some people, the relentless waves of fog that roll off the
Pacific Ocean into San Francisco each summer inspire awe. For
others, they arouse frustration, even depression. Either way,
fog is simply a fact of life for San Franciscans, particularly
those who live near the Golden Gate Bridge. The Advanced Land
Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite
acquired this view of fog encroaching on the city on August 16,
2012. The fog is part of the marine layer, a mass of cool, dense
air from the sea that was sandwiched beneath a layer of warmer
air as part of a temperature inversion. Fog is often present in
the lower part of the marine layer, whereas wispy stratus clouds
form in the upper part. Intrusions by the marine layer—and all
of the accompanying fog and clouds—are routine in San Francisco
during the summer. The intrusions are caused by westerly breezes
that push cold air inland to replace the warm air rising off of
California’s Central Valley. As it did on the day this image was
taken, the m