Flight Training - March/April 2025 - 20

PREFLIGHT WEATHER
BY DAVE HIRSCHMAN
THE COASTAL MARINE LAYER
TEMPERATURE INVERSIONS COURTESY OF THE COLD PACIFIC OCEAN
WHEN WRITER MARK TWAIN FAMOUSLY
quipped the coldest winter he ever spent
was a summer in San Francisco, he was
referring to the damp " marine layer, "
a West Coast weather feature with big
implications for pilots.
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 requirView
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March/April25
ing 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.
dave.hirschman@aopa.org
20
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