CalClim: California Climate Data Archive

CALIFORNIA CLIMATE WATCH
CURRENT ISSUE: March 2005

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The Friendly Fog

By Laura Edwards
Western Regional Climate Center

California is known for its sunny skies and warm beaches, but fog along the coast is no stranger to the locals. This region is one of the foggiest locations in the nation, with more than 60 days of dense fog reported on average per year at stations such as Los Angeles, San Diego and Eureka(1). San Francisco has an average of 1066 hours of fog per year and Point Loma near San Diego has 433 hours of fog on average. From Point Reyes to Point Arguello, six lighthouses each average more than 1000 hours per year of fog.

Figure 1. Coastal fog as seen from Trinidad Head lighthouse. http://www.rudyalicelighthouse.net/CalLts/Trinidad/Trinidad.htm

The sea fog that forms on the California coast and elsewhere around the world has been studied by scores of researchers for over a century (1,2). Worldwide, fog forecasting and climatology has been studied for any number of reasons, including routes for ships such as the Titanic, strategic war planning such as on D-Day, as well as commercial and military flight paths.

Sea fog is more common in the warm season (summer), but there is a secondary maximum in the cool season at a number of locations (3). The primary factor contributing to fog formation along the California coast is the atmospheric inversion. This is a stable atmospheric condition where there is little vertical mixing. Inversions occur often with coastal high pressure and low wind speeds, preventing the atmospheric layers from mixing. A second factor is the cool ocean water, upwelled in the California Current region. This is not to say that these are the only factors. Fog has been found to form over warm ocean water as well. The inversion factor is important because the warm, dry air and subsidence (sinking air) trap the moisture evaporating from the ocean. This condition can persist due to radiation reflecting off of the top of the fog that is formed, keeping the cloud cool and preventing evaporation. The fog normally dissipates when a north-northwesterly wind returns, or when a front or low pressure system passes through.

Lewis et al. (3) investigated fog in April 1999 and the synoptic weather patterns leading up to fog (and no fog) formation. They used back trajectories to trace the source of the air masses. They found that the large-scale circulation was similar to that of Leipper's conceptual model, with warm, dry air from the continent set the stage for fog formation. In the case of this study, subsidence was key in strengthening the inversion. Another important contributing factor to fog formation in this case was cloudiness at the top of the marine layer. In this study duration of the weather pattern was also essential in forming the fog. For some other cases that were studied, fog began to form, but did not become as widespread due to frontal passage. Thus sequential transient weather systems are essential in controlling the formation and persistence of fog along the coast.

Even though we know some characteristics of fog formation, we are far from accurate prediction in the modeling world. As Koracin et al. (4) found, similar conditions will sometimes lead to fog development where another may not. Next time you're heading to the beach, remember both your sunscreen and your jacket, as it can be just hours before the next fog rolls in.

Figure 2. Satellite image of coastal fog, June 17, 2004 00Z (5 pm local time).

References:

1. Leipper, D. F., 1995. Fog on the U.S. West Coast: a review. Bull. Amer. Meteor. Soc., 75, no. 2, 229-240.

2. Lewis, J. M., D. Koracin, K. T. Redmond, 2004. Sea fog research in the United Kingdom and United States: a historical essay including outlook. Bull. Amer. Meteor. Soc., 85, no. 3, 395-408.

3. Lewis, J. D. Koracin, R. Rabin, J. Businger, 2003. Sea fog off the California coast: viewed in the context of transient weather systems. J. Geophys. Res., 108 (D15), 4457, doi:10.1029/2002JD002833.

4. Koracin, D., J. Lewis, W. T. Thompson, C. E. Dorman, J. A. Businger, 2001. Transition of stratus into fog along the California coast: observations and modeling. J. Atmos. Sci., 58, 1714-1731.

Page last updated 4/13/05.

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Climate Watch Archive 2004 issues 2005 issues All data in the Climate Watch is provisional and subject to change. Jan 2005 [PDF] [Word] Feb 2005 [PDF] [Word] Mar 2005 [PDF] [Word] For 2004 stories, click here Monthly Summaries by Bill Mork 2005 2004	CALIFORNIA CLIMATE WATCH CURRENT ISSUE: March 2005 Read the full newsletter here: Word Format or PDF format The Friendly Fog By Laura Edwards Western Regional Climate Center California is known for its sunny skies and warm beaches, but fog along the coast is no stranger to the locals. This region is one of the foggiest locations in the nation, with more than 60 days of dense fog reported on average per year at stations such as Los Angeles, San Diego and Eureka(1). San Francisco has an average of 1066 hours of fog per year and Point Loma near San Diego has 433 hours of fog on average. From Point Reyes to Point Arguello, six lighthouses each average more than 1000 hours per year of fog. Figure 1. Coastal fog as seen from Trinidad Head lighthouse. http://www.rudyalicelighthouse.net/CalLts/Trinidad/Trinidad.htm The sea fog that forms on the California coast and elsewhere around the world has been studied by scores of researchers for over a century (1,2). Worldwide, fog forecasting and climatology has been studied for any number of reasons, including routes for ships such as the Titanic, strategic war planning such as on D-Day, as well as commercial and military flight paths. Sea fog is more common in the warm season (summer), but there is a secondary maximum in the cool season at a number of locations (3). The primary factor contributing to fog formation along the California coast is the atmospheric inversion. This is a stable atmospheric condition where there is little vertical mixing. Inversions occur often with coastal high pressure and low wind speeds, preventing the atmospheric layers from mixing. A second factor is the cool ocean water, upwelled in the California Current region. This is not to say that these are the only factors. Fog has been found to form over warm ocean water as well. The inversion factor is important because the warm, dry air and subsidence (sinking air) trap the moisture evaporating from the ocean. This condition can persist due to radiation reflecting off of the top of the fog that is formed, keeping the cloud cool and preventing evaporation. The fog normally dissipates when a north-northwesterly wind returns, or when a front or low pressure system passes through. Lewis et al. (3) investigated fog in April 1999 and the synoptic weather patterns leading up to fog (and no fog) formation. They used back trajectories to trace the source of the air masses. They found that the large-scale circulation was similar to that of Leipper's conceptual model, with warm, dry air from the continent set the stage for fog formation. In the case of this study, subsidence was key in strengthening the inversion. Another important contributing factor to fog formation in this case was cloudiness at the top of the marine layer. In this study duration of the weather pattern was also essential in forming the fog. For some other cases that were studied, fog began to form, but did not become as widespread due to frontal passage. Thus sequential transient weather systems are essential in controlling the formation and persistence of fog along the coast. Even though we know some characteristics of fog formation, we are far from accurate prediction in the modeling world. As Koracin et al. (4) found, similar conditions will sometimes lead to fog development where another may not. Next time you're heading to the beach, remember both your sunscreen and your jacket, as it can be just hours before the next fog rolls in. Figure 2. Satellite image of coastal fog, June 17, 2004 00Z (5 pm local time). References: 1. Leipper, D. F., 1995. Fog on the U.S. West Coast: a review. Bull. Amer. Meteor. Soc., 75, no. 2, 229-240. 2. Lewis, J. M., D. Koracin, K. T. Redmond, 2004. Sea fog research in the United Kingdom and United States: a historical essay including outlook. Bull. Amer. Meteor. Soc., 85, no. 3, 395-408. 3. Lewis, J. D. Koracin, R. Rabin, J. Businger, 2003. Sea fog off the California coast: viewed in the context of transient weather systems. J. Geophys. Res., 108 (D15), 4457, doi:10.1029/2002JD002833. 4. Koracin, D., J. Lewis, W. T. Thompson, C. E. Dorman, J. A. Businger, 2001. Transition of stratus into fog along the California coast: observations and modeling. J. Atmos. Sci., 58, 1714-1731. Page last updated 4/13/05.