Weather Watch

Call it ‘June gloom’ or ‘no-sky July,’ it’s cloudy on the Central Coast

A thick bank of marine layer over San Luis Obispo, viewed from the Cuesta Grade.
A thick bank of marine layer over San Luis Obispo, viewed from the Cuesta Grade.

“May gray,” “June gloom,” “no-sky July” and “Fogust” are some of the terms to describe persistent overcast conditions along the beaches of San Luis Obispo County.

And compared to last year, the coast is more overcast this season.

The clouds resemble a whitish-gray ruffled blanket that covers the coastal regions, insulating the shoreline from the afternoon and evening heat and blue sky of the North County.

So why is this coastal cloud cover so much more stubborn this year than last?

The answer is probably El Niño and here’s why: Last year, seawater temperature along the Pecho Coast — the coastal stretch between Point San Luis Lighthouse and Point Buchon just south of Montaña de Oro State Park — was the warmest on record. The last two warmest years occurred during the strong El Niño events of 1983 and 1998.

Warm coastal waters don’t create the conditions for fog. That takes the colder seawater we are experiencing this year, now that El Niño has gone.

The conditions start with the relentless gale-force northwesterly (onshore) winds we often get in the spring with gusts of more than 50 mph. These winds produce heavy upwelling along the coast, bringing cold, subsurface water to the surface along the immediate shoreline.

As you head farther out to sea, the surface water becomes warmer.

When the northwesterly winds relax, like last week, milder 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. If the seawater is warm like last year, the overlying air may not cool enough to reach its dew-point temperature and will not condense into clouds. In other words, the warmer seawater usually contributes to less fog along the shoreline.

Fog is made up of tiny water droplets suspended in the air column only about twenty-five-thousandths of an inch 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, you can often see wet streets and sidewalks under the pine trees and in our coastal communities during the summer.

Another factor that contributed to less fog last year was the great amount of monsoon moisture.

One year ago, abundant subtropical moisture from the remnants of Hurricane Dolores produced record amounts of July rainfall in San Luis Obispo County. Previous records for the month of July were .59 of an inch at Paso Robles and .46 of an inch in San Luis Obispo.

One year ago, a weather station in Paso Robles reported 3.55 inches, while numerous weather stations around San Luis Obispo reported about an inch of rain. Accompanying the rain were approximately 35,000 lightning strikes, according to www.SLOweather.com. This subtropical moisture tends to result in much less fog.

Gale-force northwesterly winds will return along the coastline this week, producing much greater amounts of sunshine along the beaches, while the North County will reach triple digits just in time for the California Mid-State Fair in Paso Robles.

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PG&E conservation tips: Mid to high temperatures are expected in the North County this week. When temperatures rise, it can be a challenge to keep your water and energy use down. Please visit www.pge.com for a list of energy conservation tips. They will help you reduce your usage and save you money while keeping you cool and comfortable all summer long.

John Lindsey is PG&E’s Diablo Canyon nuclear power plant marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John for weather and other useful information.

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