The coastal marine layer

This stratus layer of low clouds and sometimes fog forms when warm air interacts with the chilly Pacific Ocean, cools to its saturation point, and creates a temperature inversion. Cool, humid air near the ocean surface becomes opaque, and then it gets pushed against the coastline by onshore sea breezes.

Warm, dry air above the marine layer “traps” this cool air at the surface, and it’s known to stick around for days at a time. The resulting “May gray” or “June gloom” often extends miles inland, especially at night or during the early morning hours. The marine layer can burn off during the day when sunshine cuts through the clouds and warms the land beneath. Then the clouds reform after sunset when surface temperatures drop again.

The marine layer is usually relatively thin with tops at about 2,000 feet, but bases and visibility can be very low requiring IFR approaches to coastal airports. And sometimes even precision approaches aren’t low enough for pilots to safely get below the clouds and see the runway.

The marine layer is most prevalent in Southern California, yet it can extend all the way up the West Coast during summer. California pilots account for it by flying VFR during the middle of the day when the marine layer recedes or disappears completely. Otherwise, IFR departures or approaches are the norm.

Busy Class B airports such as San Diego International Airport (SAN), Long Beach (LGB), and Los Angeles International Airport (LAX) are subject to the marine layer as well as thick fog. General aviation airports such as Torrance (TOA), Oxnard (OXR), and Camarillo (CMA) are subject to the same conditions.

Santa Barbara (SBA) and Monterey (MRY) farther up the coast have the added complications of mountainous terrain as well as clouds that can move in quickly, as do many of the airports in the San Francisco Bay Area.

Monterey can be particularly tricky since the ILS and RNAV approaches to Runway 10R begin over the ocean and sometimes involve a significant tailwind during the descent and landing. A missed approach requires an immediate climbing turn to avoid tall terrain. Pilots commonly must descend to a decision height of about 200 feet before they can see the runway and land. Yet they’re only in the clouds for about two or three minutes during the approach itself because the sky above the marine layer usually is clear with excellent visibility.

“The temperature inversion makes the air above the marine layer extremely smooth,” said Steve Colburn, a California pilot who lived and flew extensively in the Monterey Bay area. “Then you enter the clouds, and visibility drops all at once and water beads on the windshield. You do your best to keep the needles centered until you break out near minimums. And even though you’re only in the clouds for a short time, you’ve got to be on your game because you’re quite close to the ground.”

Marine layers aren’t exclusively found on the West Coast. They also can form around the Great Lakes and the Atlantic. Also, the same factors of warm air over cold water that create temperature inversions can be found in river valleys everywhere.

When flying over West Virginia or New England on an autumn morning, for example, the tendrils of fog in the valleys or over expansive lakes are mini-marine layers: warm air that interacts with cold water, becomes saturated, and produces clouds with nowhere else to go.

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