The marine layer is fog that rolls in from the ocean, where it develops, to hang out for an annoyingly long time on what would otherwise be a beautiful day. It’s an air mass that develops over the surface of a large body of water, in the presence of a temperature inversion—which is usually initiated by the cooling effect of the body of water on the surface layer of an otherwise warm air mass. The cooling air becomes denser than the air above it and gets stuck below that air. Coastal communities will be blanketed with advection fog until the sun is strong enough to evaporate it, hence the colloquialism, “Burn it off.” Typically marine layers have bases of 400 to 800 feet and tops of 3,000 feet.
“The ocean is like the freon in your refrigerator in the summer,” said CFII Jason Miller, host of The Finer Points aviation podcast, who flight instructs in the San Francisco Bay area. “When the summer hits and the state gets hot, the Valley hits 110 degrees; that heat draws all the air inland. That becomes the sea breeze effect. As the air comes in at night, the land cools down dramatically and the air is so moist, it comes over the cold land and turns it into this blanket of fog.”
Miller said the microclimate creates excellent conditions for instrument training and flying. During this time of year, pilots need to be mindful not to get stuck on top of a marine layer. An instrument-rated and proficient pilot can file a pop-up IFR clearance to land at a fogged-in airport, but that’s not legal for VFR pilots, who would need to scurry inland and land at another airport reporting VFR conditions.
If the marine layer is being particularly stubborn about moving off, a special VFR clearance could help. Special VFR clearances permit pilots to take off or land in controlled airspace when conditions are less than VFR. California pilots know that in some instances, an airport may report “broken” conditions, but the weather is clear to the east with a fog bank to the west. Bear in mind that some airports don’t permit fixed-wing aircraft to conduct SVFR operations, and if you are not familiar with the characteristics of the marine layer, don’t make flying plans—especially fuel-related decisions—based on when you think the marine layer will dissipate.