The Foggy Season
in the San Joaquin Valley


"The fog comes on little cat feet. It sits looking over harbor and city on silent haunches and then moves on".
FOG - Carl Sandburg - 1878

If one were to describe the most notable weather characteristics of the San Joaquin Valley, undoubtedly they would include the occurrence of dense fog episodes during the winter. Many who move to the interior of Central California seem to express surprise at the persistence and intensity of the fog that can develop during the winter months. Fog Season is one of the most dangerous parts of the year in Central California. Associated with the rainy season in the San Joaquin Valley the dense fog is almost a daily occurrence from mid-October to early March.

Before one can understand why Central California has such a problem with fog, one must first understand the climate that supports the phenomenon and the geography of the land. The San Joaquin Valley stretches from the Sacramento- San Joaquin Delta to the North Tehachapi Mountains. The Valley has the various coastal ranges to the west and the Sierra Nevada to the east. In conjunction with the Sacramento Valley the Great Valley of California is essentially a closed air basin. The climate in Central California is one of long hot dry summers, occasional blowing dust events in the fall and spring and cool winters with dense fog. The introduction of moisture is not removed from the air basin unless pushed or lifted out by a weather event. In the fall the cool season begins and more frontal systems begin to move in and bring the much needed rain to Central California. After the rain, low level moisture is added thereby setting the stage for fog to form.

Fog - as defined in the encyclopedia, 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 called 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 the night is long, 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 (condensation nuclei) to cling to and the valley usually has plenty of that due to pollution and dust. So the cool air lies near the valley floor and fog forms throughout the night. The temperature inversion layer can also play a roll in fog formation. The inversion layer, warmer temperatures above cooler temperatures, also sets up over a period of time and that can eventually lead to a layer of fog from about 600 feet to 3000 feet above the surface. This sometimes allows the fog to form in the Foothills of the Mountains while the Valley floor is cold and dreary with peeks of sunshine. The lack of strong sunshine during daytime hours does not provide sufficient incoming energy to always evaporate the overnight fog, thus fog can and does last several days at a time. The atmosphere needs additional drying or mixing of its air basin for the fog to lift out.

One form of radiation fog that is the leading cause of weather-related casualties in Central California is Tule fog. This thick ground fog condenses when there is a high relative humidity, which usually comes after a heavy rain, light winds and rapid over night 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 a mere foot. 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 pile-ups on the highways and roads in Central California.

In the morning hours of dense fog on November 14th 1998 reports began pouring into the California Highway Patrol at 8:50-AM of a multiple vehicle pile-up on highway 99. More than 74 vehicles, including 19 tractor trailers were involved in the one mile stretch just two miles southeast of Kingsburg, CA. there were two fatalities and 51 injuries. Over a 132 people were involved in the accident. On the same day another eight accidents occurred in the Fresno area as well as an eight car pile-up which occurred just two miles away from the accident near Kingsburg. The accident was recorded as the worst event until the winter of 2007. On November 1st, 2007 a 108 car pile up occurred on Highway 99 in Fresno killing three people and injuring over 100. The pile-up was later blamed on a drunk driver going to fast in the morning fog causing a change reaction accident.

Forecasting Fog is difficult to predict but is easier during the months of December and January. It becomes harder to predict just before and near the end of winter. The difficulty lies with predicting widespread or patchy dense fog. If the weather pattern is extremely stagnant, then widespread dense fog is a good bet. Studies are being done on better handling of fog prediction using the 700 millibar computer models. According to National Weather Service Meteorologist and Author Mark Burger "Forecasters often rely on inspection of low level winds, the presence of a large-scale anticyclone aloft, initially clear skies, damp surface conditions, and climatology to gauge the potential for dense fog in the San Joaquin Valley. However, many of these methods are quite subjective in nature and/or are overly reliant on model guidance, as opposed to observational data. Thus, my study seeks to objectively examine one parameter, upper level heights derived from radiosonde data, and correlate its effectiveness in diagnosing and predicting dense fog across the central San Joaquin Valley". The advancement of Satellite images from the National Weather Service and ground level cameras provided by the CHP, non-traditional technology is helping in showing the areas of fog.

Fog patterns in Central California have not changed in the past few years. The thickness, duration and amount of days all depends on how much rain the Valley receives and how strong the high is over the Valley after the rain. The San Joaquin Valley averages about 36 days of dense fog every winter when the visibility is a quarter of a mile or less. But when the Valley's weather pattern is active like in an El Nino pattern, the air can remain to mixed for fog to form. If it does form, a succeeding storm would prevent the fog from reforming and if the atmosphere simply remains to mixed, the air dries and fog has a tougher time forming.

Therefore, with the combination of geography, weather conditions and scientific data, we can better prepare for the dense fog in the San Joaquin Valley.

1. Mark Burger - The Application of Upper Level Heights in Diagnosing and Forecasting San Joaquin Valley Dense Fog Episodes
2. Tule Fog
3. Highway Fog related accidents
4. Fog
5. Fog Season
6. California Highway Patrol
7. San Joaquin Valley
8. Fog and Speed Kills
9. Carl Sandburg "Fog" Poem