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Tule fog () is a thick ground
fog Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...
that settles in the
San Joaquin Valley The San Joaquin Valley ( ; es, Valle de San Joaquín) is the area of the Central Valley of the U.S. state of California that lies south of the Sacramento–San Joaquin River Delta and is drained by the San Joaquin River. It comprises seven ...
and
Sacramento Valley , photo =Sacramento Riverfront.jpg , photo_caption= Sacramento , map_image=Map california central valley.jpg , map_caption= The Central Valley of California , location = California, United States , coordinates = , boundaries = Sierra Nevada (ea ...
areas of
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
's Central Valley. Tule fog forms from late fall through early spring (California's
winter Winter is the coldest season of the year in polar and temperate climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Different cultur ...
season) after the first significant rainfall. The official time frame for tule fog to form is from November 1 to March 31. This phenomenon is named after the tule grass wetlands (''tulares'') of the Central Valley. Tule fog is the leading cause of weather-related accidents in California.


Formation

Tule fog is a
radiation fog Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...
, which condenses when there is a high
relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
(typically after a heavy rain), calm winds, and rapid cooling during the night. The nights are longer in the winter months, which allows an extended period of ground cooling, and thereby a pronounced
temperature inversion In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to an inversion of the air temperature lapse rate, in which case it is called a temperature inversion. Nor ...
at a low altitude. In California, tule fog can extend from
Bakersfield Bakersfield is a city in Kern County, California, United States. It is the county seat and largest city of Kern County. The city covers about near the southern end of the San Joaquin Valley and the Central Valley region. Bakersfield's populat ...
to Red Bluff, covering a distance of over . Tule fog occasionally drifts as far west as the
San Francisco Bay Area The San Francisco Bay Area, often referred to as simply the Bay Area, is a populous region surrounding the San Francisco, San Pablo, and Suisun Bay estuaries in Northern California. The Bay Area is defined by the Association of Bay Area G ...
via the Carquinez Strait, and can even drift westward out through the Golden Gate, opposite to the usual course of the coastal fog. Tule fog is characteristically confined mainly to the Central Valley due to the mountain ranges surrounding it. Because of the density of the cold air in the winter, winds are not able to dislodge the fog and the high pressure of the warmer air above the mountaintops presses down on the cold air trapped in the valley, resulting in a dense, immobile fog that can last for days or at times for weeks undisturbed. Tule fog often contains light drizzle or freezing drizzle where temperatures are sufficiently cold. Tule fog is a low cloud, usually below in
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
and can be seen from above by driving up into the
foothills Foothills or piedmont are geographically defined as gradual increases in elevation at the base of a mountain range, higher hill range or an upland area. They are a transition zone between plains and low relief hills and the adjacent topogr ...
of the
Sierra Nevada The Sierra Nevada () is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primar ...
to the east or the
Coast Ranges The Pacific Coast Ranges (officially gazetted as the Pacific Mountain System in the United States) are the series of mountain ranges that stretch along the West Coast of North America from Alaska south to Northern and Central Mexico. Although ...
to the west. Above the cold, foggy layer, the air is typically mild, dry and clear. Once tule fog has formed, turbulent air is necessary to break through the temperature inversion layer. Daytime heating (cloud-penetrating visible light wavelengths transformed to infrared by the ground) sometimes evaporates the fog in patches, although the air remains chilly and hazy below the inversion and fog reforms soon after sunset. Tule fog usually remains longer in the southern and eastern parts of the Central Valley, because winter storms with strong winds and turbulent air affect the northern Central Valley more often.


Visibility

Visibility in tule fog is usually less than an eighth of a mile (about 600 ft or 200 m). Visibility can vary rapidly; in only a few feet, visibility can go from to near zero. The variability in visibility is the cause of many chain-reaction pile-ups on roads and freeways. In one such accident on Interstate 5 near Elk Grove south of
Sacramento ) , image_map = Sacramento County California Incorporated and Unincorporated areas Sacramento Highlighted.svg , mapsize = 250x200px , map_caption = Location within Sacramento ...
, 25 cars and nine big-rig trucks collided inside a fog bank on December 12, 1997. Five people died and 28 were injured. It took 26 hours to clear away all the wreckage and reopen the freeway. In February 2002, two people were killed in an 80-plus-car pile-up on State Route 99 between Kingsburg and Selma. On the morning of November 3, 2007, heavy tule fog caused a massive pile-up that included 108 passenger vehicles and 18 big-rig trucks on northbound State Route 99 between Fowler and Fresno. Visibility was about at the time of the accident. There were two fatalities and 39 injuries in the crash.


Freezing drizzle and black ice

Tule fog events are often accompanied by drizzle. Sunlight cannot penetrate the fog layer, keeping temperatures below freezing. Episodes of freezing drizzle occasionally accompany tule fog events during winter. Such events can leave an invisible glaze of black ice on roadways, making travel especially treacherous.


Composition

The composition of tule fog in the San Joaquin and Sacramento valleys consists mainly of ammonia, nitrate and sulfate concentrations. Furthermore, ammonia is the most commonly found single ion and usually is measured to be more than half of the measured ions in the fog. Depending on the region within the California Central Valley, the composition of tule fog can vary in element or ion concentrations. As of 2014, it has been discovered that the quantity of tule fog has decreased in the Central Valley from when it was initially studied from 1981-1999 compared to 2001-2012. The frequency of tule fog occurrence is proportional to the higher air pollution in California. Minimum temperature, DPD (the difference between ambient temperature and dew point), precipitation, and wind speed are the four major components that affect fog formation. Minimum temperature affects tule fog formation because it is an extreme form of radiation fog that most often forms after sunset due to rapid surface radiative cooling. Low DPD is consistent with more frequent periods of fog. Precipitation has somewhat of a correlation to an increase in fog however it is not directly correlated due to some precipitation totals being inversely correlated to some fog years. Wind speed has a small but statistically important impact on fog frequency being that lower wind speeds are correlated with higher fog frequency. Winter causes the optimal meteorological conditions for fog formation due to periodic storms followed by long periods of high pressure throughout California.


References


External links


Page on tule fog
from the National Oceanic and Atmospheric Administration (NOAA)
University Corporation for Atmospheric Research: Forecasting Radiation Fog

Davis, CA Tule fog
— ''featured in
Orion Magazine ''Orion'' is a quarterly, advertisement-free, nonprofit magazine focused on nature, culture, and place addressing environmental and societal issues. It has published such authors as Wendell Berry, Barry Lopez, Terry Tempest Williams, Michael Po ...
''. {{DEFAULTSORT:Tule Fog Fog Natural history of the Central Valley (California) Climate of California Weather events in the United States