Following the storm system of two weeks ago, fog has returned with a vengeance to the San Joaquin Valley. Tule fog, the infamous fog that plagues these low-lying regions in the winter, is the leading cause of weather-related casualties in Central California.
This past week, this persistent fog has been creeping into Three Rivers. This community is a refuge for residents and Valley dwellers because it's above the fog. It's a place known for its sunny days while lower elevations are swallowed by the dreary gray muck.
Fog is a cloud of condensed water vapor droplets or ice crystals, suspended in the atmosphere just over the surface of the Earth. In simple terms, fog is a cloud in contact with the ground.
However, this "cloud" is different than any other and comes in many shapes and forms. The main fog that forms in the San Joaquin Valley is radiational fog.
First, a good rainfall covers much of Central California, then high pressure builds behind the passing storm and settles the atmosphere. Once the air is quiet and nightfall arrives, clear skies and light wind allow heat gained during the day to radiate out.
The temperature falls to the dew point and the air near the surface becomes saturated. Fog needs some microscopic particles to cling to and the Valley usually has plenty of that due to pollution, smoke, and dust. So the cool air settles in near the Valley floor and fog forms throughout
The temperature inversion layer can also play a roll in fog formation. The inversion layer, warmer temperatures (such as in Three Rivers) above cooler temperatures (Visalia), also sets up over a period of time, and that can eventually lead to a layer of fog from about 600 feet to 3,000 feet above the surface.
This is what Three Rivers has experienced during some mornings over the past week, when the warmer temperatures have allowed the fog to form in the Sierra foothills while the Valley floor is cold and dreary with glimpses of sunshine. The lack of strong sunshine during winter's daytime hours does not provide sufficient incoming energy to always evaporate the overnight fog, thus fog can and does linger for several days, even weeks, at a time. The Valley experiences an average of 36 foggy nights each winter.
To get rid of the fog, the atmosphere needs additional drying or mixing of the air basin for the fog to lift out.
The Valley's Tule fog is a thick ground fog that condenses when there is a high relative humidity, which usually comes after a heavy rain, light winds, and rapid overnight cooling. Tule fog forms when the cold mountain air flows into the valley during the night, pooling in the low areas and filling the valley to the brim.
Because of the density of the cold air, winds are not able to dislodge the fog and the high pressure of the warmer air above the mountains presses down on the cold air and traps the fog in the valley. The result is a dense immobile fog that reduces visibility to mere feet at times.
The Tule fog usually develops below 1,000 feet, and above the layer of cold air is typically a layer of warmer, dry, clear air. Once the fog has formed, strong turbulent air is necessary to break through the temperature inversion layer.
Daytime heating can sometimes help evaporate the fog. There is not much that one can see in Tule fog.
Visibility is usually reduced to a few hundred feet but can sometimes be less than 10 feet. Visibility in Tule fog varies rapidly and is the cause of many chain reaction vehicle pile-ups on highways and roads in Central California.