marine layer

The cool, moist blanket of air that forms over cold ocean water and rolls onshore along the Pacific coast, producing the low stratus clouds and fog that define the coastal experience from Baja to British Columbia. The marine layer can be a few hundred feet thick or a few thousand. When it's deep, the coast is socked in; when it's shallow, hilltops poke through into sunshine while the valleys stay gray. "June gloom" is the marine layer at its most persistent. It burns off by noon or it doesn't, and the day is decided by which.