Weather Watch

When strong winds abate, fog moves in

Special to The TribuneMay 25, 2013 

Marine low clouds and fog stream past Valencia Peak in Montaña De Oro State Park on April 28.

JOHN LINDSEY

The relentless gale force northwesterly (onshore) winds with gusts of more than 50 mph at the Diablo Canyon meteorological tower has produced heavy upwelling along the coast. This phenomenon brings cold, subsurface water to the surface along the immediate shoreline.

Seawater temperatures have plummeted from 59 degrees a few weeks ago to a numbing 48 degrees today. As you head farther out to sea, the surface water becomes warmer.

When these northwesterly winds finally relax somewhat, we could be in for some dense fog. You see, the northwesterly winds transport the relatively warm air from farther out to sea across the much colder water along our immediate coastline.

The overlying air then becomes chilled and drops to its dew-point temperature, producing a wall of gray. The type of fog we see along our rugged coastline is advection fog. Advection simply means transport.

Fog is made up of tiny water droplets suspended in the air column only about 0.025 inches in diameter. There may be thousands of droplets in 1 cubic inch of air. Depending upon atmospheric conditions (pressure, temperature and relative humidity), a cubic mile of fog may hold millions of gallons of water.

If the onshore winds are between gentle to fresh (8- to 24-mph) levels, the mist from the Pacific can sweep in like a ghostly blanket moving through the trees. Tiny fog droplets stick to the leaves or pine needles where they clump together and form large drops that fall to the ground. When the conditions are right, it is not unusual to see wet streets and sidewalks under the pine and eucalyptus trees in our coastal communities during the summer.

Strong to gale force (25- to 38-mph) or greater northwesterly winds can often mix out the temperature inversion layer, producing mostly clear skies, while light to gentle (1- to 7-mph) onshore winds can often produce a stubbornly persistent marine layer that can last throughout the day along the entire coastline.

David Martin, who lives in Morro Bay, developed an interesting acronym to describe the marine overcast: “LOCCIA.” It stands for “Low-Over-Cast-Clear-In-Afternoon.” If it’s a light LOCCIA it will clear by 10 a.m. A medium condition will clear by 2 p.m., and a heavy LOCCIA will clear by 5 p.m. — if at all.

• • •

Meaningful Memorial Day observances are planned throughout the county Monday to honor the military members who sacrificed everything to preserve our freedom.

One is the Lost at Sea memorial at the Cayucos Pier. In 2001, Pastor Doug Carroll, Joe Eyeraud, Navy Chaplain Bill Houston, Dave Congalton, Bill Benica and Tom Madsen created the Memorial Day service to honor all those lost at sea: military, recreational and commercial.

We gather at the base of the pier for a service by Houston at 3 p.m. and walk together out over the Pacific Ocean, remembering those who never returned.

John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist. If you have a question, send him an email at pgeweather@pge.com.

The Tribune is pleased to provide this opportunity to share information, experiences and observations about what's in the news. Some of the comments may be reprinted elsewhere in the site or in the newspaper. We encourage lively, open debate on the issues of the day, and ask that you refrain from profanity, hate speech, personal comments and remarks that are off point. Thank you for taking the time to offer your thoughts.

Commenting FAQs | Terms of Service