As anyone who has lived along the Central Coast for any length of time will tell you, July tends to feature a lot of night and morning low clouds and fog along the beaches.
Unfortunately, the fog can threaten the enjoyment of Fourth of July fireworks displays in Cayucos, Morro Bay and Pismo Beach. All it takes is a small change in air temperature, the temperature inversion layer or the direction or speed in the wind to make the difference between a beautiful fireworks display versus a light show in the coastal stratus.
Even if the marine layer may roll in Monday night, the base of its clouds may be high enough to still allow for a nice viewing of the firework displays. Fireworks are made to burst at different heights, depending on the size of the shell and how the fireworks are packed. Most firework displays along our coastline reach an altitude between a few hundred to 1,000 feet.
Fog is made up of tiny water droplets suspended in the air column only about a twenty-five thousandth of an inch in diameter. There may be thousands of droplets in one 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.
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The type of fog we most commonly see along our rugged coastline is advection fog. Advection simply means transport. Advection fog is carried by the wind wherever it may go.
The persistent northwesterly (onshore) winds produce upwelling along the coast, which brings cold, subsurface water to the surface along the immediate shoreline. As you head farther out to sea, the surface water becomes warmer. The northwesterly (onshore) 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 coastal low clouds and fog. If the wind direction is out of the northeast (offshore), clear skies will develop.
Another important aspect of fog development is topography. The coastal mountains of San Luis Obispo County force the northwesterly moisture-laden winds blowing horizontally from the Pacific Ocean to turn vertical, or upward. As the air mass is lifted up over our coastal mountains (upwind), it cools and eventually reaches its dew point temperature. You can often see this at Montaña de Oro State Park.
On the leeward (downwind) side of the mountain, the air mass is forced downward by gravity and warmed by pressure (compressional heating). The sinking air is drier after losing some of its moisture on the upwind side of the mountain range. This is why our southwesterly facing beaches such as Cayucos and Avila Beach often see more sun. If the winds shift out of the south, the opposite can occur. The southwesterly facing beaches will struggle with the coastal stratus while the northwesterly facing beaches (Los Osos and Morro Bay) will be mostly clear. Strong to gale force northwesterly winds can often mix out the temperature inversion layer, producing mostly clear skies, while calm to light winds can often produce a stubbornly persistent marine layer that can last throughout the day along the entire coastline.
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.
A nonprofit group from Canada called FogQuest constructs fog collectors that produce fresh water for communities in arid and rural regions of the world where normal water supplies are not available. Their fog collectors resemble large volleyball nets made of plastic mesh material stretched perpendicular to the prevailing winds on mountain slopes. One of their large fog collectors, with a 130-square-foot surface, can produce an average of 53 gallons of freshwater per day through the year, with up to 265 gallons on good days and none on other days.
For more information, visit www.fogquest.org.
Strong high pressure across California will continue to produce hot North County temperatures today with many locations north of the Cuesta Grade (Paso Robles) ranging between 104 and 108 degrees.
On the south side of the Cuesta Grade, fresh to strong (19 to 31 mph) northwesterly (onshore) winds will keep the coastline cool. The coastal valleys (San Luis Obispo) will be mostly in the mid-80s.
The marine layer will blanket the immediate coastline. The northwesterly facing beaches (Montaña de Oro, Los Osos and Morro Bay) will range between the high 50s to the mid-60s while the southwesterly facing beaches (Cayucos and Avila Beach) will range between the high 60s to the low 70s.
For Independence Day, it continues to look like a coastal stratus will stubbornly persist along the immediate coastline with the cloud ceiling near 900 feet for most of the day. This condition will produce gradually cooler temperatures throughout the Central Coast.
A strong area of high pressure over the Four Corners area will also direct some monsoon moisture from the desert Southwest toward the Central Coast, producing higher humidity levels and cumulus clouds over the eastern coastal mountain ranges of the county.
Strong high pressure will keep temperatures in low 100s in the North County and near 110 across the southern San Joaquin Valley on Tuesday.
It appears that the county will cool in earnest Wednesday and Thursday, with increasing coastal low clouds and afternoon (onshore) northwesterly winds flowing through the Templeton Gap. Increasing northwesterly (onshore) winds will produce widespread cooling Friday with temperatures falling back to near normal by next weekend.
Today’s fresh to strong (19 to 31 mph) northwesterly (onshore) winds will generate a 5-to-7-foot northwesterly (310-degree deepwater) sea and swell (with a 5-to-12-second period) today through Monday.
The wind fields will shift northward off the Northern California coastline leaving behind 4-to-6-foot northwesterly (300-degree deepwater) swell (with an 8- to 10-second period) Tuesday.
This northwesterly swell will decrease to 3 to 5 feet (with an 8- to 10-second period) Wednesday through Friday morning.
Increasing northwesterly winds will produce a 4- to 6-foot northwesterly (315-degree deepwater) sea and swell (with a 5- to 9-second period) Friday through Saturday.
Seawater temperatures will range from 54 to 56 degrees through Monday, increasing to 55 to 57 degrees Tuesday through Friday.
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