Airports within a few miles of the ocean cancel and delay more summer morning flights than fields fifty miles inland. The cause is a specific kind of fog that forms over water rather than over land.
Marine fog forms over water, not over the runway
Warm, moist air moving across a cooler sea surface is chilled from below until it reaches saturation. The result is a shallow deck of cloud sitting on the water, often only a few hundred feet thick.
Because the cooling comes from the sea rather than from the ground losing heat overnight, this fog does not need a clear calm night to form. It can arrive on a breezy afternoon and persist for days.
Inland radiation fog behaves differently. It forms in place on cold clear nights and usually burns off within an hour or two of sunrise, which is why inland airports recover earlier.
The sea breeze delivers it to the airfield
As land heats through the morning, air rises over it and cooler marine air is drawn inland to replace it. That circulation carries the fog bank with it, sometimes several miles past the beach.
An airport sitting in that path can go from clear to obscured in minutes. Crews watching the field from the terminal often see the deck arrive as a visible wall moving up the coastline.
Low visibility changes how many aircraft can land
When visibility and ceiling drop, controllers increase spacing between arriving aircraft because pilots need more time and distance to establish on an instrument approach. Fewer aircraft per hour can be accepted.
Airports typically publish that reduced acceptance rate to the wider system, and flights bound for the field are held at their origin rather than circling. A delay that begins at the coast surfaces at airports far away.
Some coastal fields also have approaches limited by terrain or by the equipment installed on a given runway, so the usable options narrow further in poor conditions.
Why the morning bank suffers most
Marine layers are usually deepest and most extensive around dawn, then thin as the land warms and mixing increases. That timing overlaps with the heaviest departure and arrival push of the day.
An aircraft stuck on the ground in the morning is the same aircraft scheduled for three more legs. The knock-on effect through the rest of the day is larger than the original delay.
What a traveler can read before booking
Coastal fog is seasonal and local, and the pattern at one airport can be markedly different from another an hour up the same shoreline. Local climate summaries describe those differences well.
Where a coastal field has a persistent summer fog season, an inland alternate airport with a short drive to the beach can be the more reliable connection point.