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the marine layer as well as evaporation of any clouds within it. The marine layer clears back toward the coast, usually by noon. By mid-afternoon, inland areas have heated sufficiently to decrease the air pressure and increase the onshore flow. By late afternoon, the wind increases and begins to cool the onshore marine layer, allowing the fog and low clouds offshore to progress inland without evaporating. Cloudiness streams in over the Bay and through the various gaps. How far the clouds can penetrate inland depends on the depth of the marine layer and the strength of the cooling winds. As night falls and inland areas cool down, the winds usually decrease, but the fog and clouds remain wherever they have blown in until the following morning when the cycle repeats.
233:
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166:. In the summer, inland temperatures can soar above 100 °F (38 °C). This large difference creates a strong pressure gradient that turns the prevailing northwest flow to a westerly and even southwesterly direction near the coastline, driving the marine layer and its clouds onshore and through any gaps in the
189:
Under normal summertime conditions, a daily pattern of fog and low clouds occurs. Morning sun light heats the ground (cloud-penetrating visible light wavelengths transformed to infrared by the ground), which in turn heats the marine layer over the land areas. This creates convective turbulence within
99:
contributes to the frequency of fog by providing atmospheric moisture and by its temperature. It is also the major source of nuclei for the condensation of moisture from vapor into cloud droplets. Moisture evaporated from the ocean surface over hundreds, even thousands of miles of the open
Pacific is
202:
Another pattern variation occurs in connection with heat spells that reach the coast from inland. Such heat waves typically occur when an area of high atmospheric pressure orients itself in such a way that the northerly to northeasterly gradient becomes dominant, driving the marine layer out to sea
221:
There are also occasional extended spells when fog and stratus ("overcast") do not clear all the way back to the coast for several days. These extended periods of cloudiness are usually a consequence of a weak area of low pressure above the marine layer which increases its depth, making it more
162:. As the coastline is oriented from northwest to southeast, the marine layer and any clouds present within it would be confined to the coast and adjacent offshore waters, and often are, but for the large difference in temperature between the coastal waters and the inland valleys, especially the
185:
and the
Central Valley. As the city of San Francisco lies adjacent to the Golden Gate, it is often subject to the fog and low clouds blowing in on the marine layer. Even when the clouds are not present, the coolness of the marine layer exacerbated by the strong winds can chill the city even in
239:
198:
A land/sea temperature-pressure gradient is not always necessary to drive the marine layer and low clouds onshore into the Bay Area. Winds ahead of an approaching cold front or low-pressure system can also push the marine layer onshore.
75:, can occur during the winter. There are occasions when both types can occur simultaneously in the Bay Area. The prevalence of fog in the San Francisco Bay Area has decreased, and this trend is typically attributed to
232:
263:. Lower fog levels are also problematic for the agricultural regions fog patterns support, such as the Napa and Salinas Valleys. The attribution of the reduction in fog and of global warming itself to the
363:
connecting San
Francisco and Oakland. In 2013, the Overseas Reymar, a 748-foot-long tanker, was proceeding to sea from an anchorage off San Francisco after discharging its cargo of crude oil at a
214:
above the marine layer inversion can, depending on its severity, break up the marine layer. The most common causes of such turbulence are strong upper-level low pressure areas, or the
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691:
130:
are present, liquid water drops will form. Condensation nuclei in coastal fog are mostly composed of salt from surf and spray, with lesser amounts of iodine from
778:
606:
255:
Research published in 2010 showed that summertime fog in
California decreased by 33% during the 20th century. The decline in fog is generally attributed to
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difficult for surface heating to evaporate the clouds within it. These periods of persistent overcast have inspired colloquialisms such as "May Gray", "
1053:
761:
483:
446:, NOAA Technical Memorandum NWS WR·126, Jan Null, National Weather Service Forecast Office, San Francisco Bay Area, California, January, 1995
851:
360:
1043:
568:, Ismail Gultepe, Reprint from Pure and Applied Geophysics, Vol. 164 (2007), No.6-7, Springer Science & Business Media, Jan 2, 2008
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461:
31:
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is due to a combination of factors particular to the region that are especially prevalent in the summer. Another type of fog,
1015:
903:
535:
Rand, McNally & Co.'s New
Overland Guide to the Pacific Coast: California, Arizona, New Mexico, Colorado, and Kansas
111:
flows from the northwest and is cool owing to its origin in the North
Pacific. Additional cooling occurs due to strong
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264:
163:
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carried to
California from various directions. This water vapor contributes to the development of what is called a
142:
692:"'Fog City' by Documentary Filmmaker Sam Green Premieres October 2-3, 2013 with Live Performances by The Quavers"
60:
626:
579:
Weather Aloft, Angela Spivey, Endeavors
Magazine, University of North Carolina at Chapel Hill, September 1, 1997
484:"FOG HEAVEN / The sun will come out tomorrow. Or maybe not. It's summer in the city, and that means gray skies"
442:
658:
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167:
134:. These nuclei are so effective that condensation can occur even before the relative humidity reaches 100%.
116:
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52:
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The prevailing wind along the
California coast is from the northwest owing to the normal location of the
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397:
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of cooler subsurface waters, especially along the immediate coastline and near various promontories.
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507:
207:, when the northerly gradient relaxes, allowing the marine layer to "slosh back" up the coastline.
127:
186:
mid-summer. Because of this, San
Francisco is sometimes described as "naturally air conditioned".
87:
284:
155:
108:
44:
386:"Climatic context and ecological implications of summer fog decline in the coast redwood region"
122:
When the marine layer encounters the colder waters along the California coast, it cools to its
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Many accidents happened because of the fog. In 2007, in fog, a container ship struck the
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south and west of the California coast. These spells typically end with what is called a
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385:
256:
76:
351:. The name was disparaged by 3 local celebrities during the 2020 PGA golf tournament.
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974:
695:
296:
288:
96:
68:
1048:
852:"PGA Championship Twitter account refers to SF fog as 'Karl,' incensed locals rage"
817:"Karl the Fog upstages the Salesforce Tower opening, and social media gets a laugh"
335:
that year, and named "Karl the Fog" after the misunderstood giant in the 2003 film
280:
260:
101:
779:"Capital Weather Gang: Pic of the week: San Francisco's fog has a name. It's Karl"
726:"Behind the Tweets: The Secretive People Behind S.F.'s Fog, Seagulls, and Bridges"
533:
307:, San Francisco's fog becomes a metaphor for grief and the limitations of memory.
590:"The Pacific Coastal Fog Project: Developing ecologically relevant fog datasets"
300:
275:
Writers, poets, and photographers have long been inspired by the fog, including
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which occasionally extends northwestward from the desert areas of the U.S.
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refinery when it hit the base of the Echo tower of the Bay Bridge in fog.
337:
123:
72:
1027:
878:"Container ship hits Bay Bridge tower - fuel spills, but span undamaged"
119:
along the coast are generally 52–58 °F (11–14 °C) year-round.
324:
659:"The Pacific Decadal Oscillation (PDO) is not causing global warming"
589:
834:"Let's make a decision, San Francisco: Are we calling the fog Karl?"
508:
Stressed seaweed contributes to cloudy coastal skies, study suggests
876:
Taylor, Michael; Nolte, Carl; Curiel, Jonathan (November 8, 2007).
210:
Yet another variation occurs when the upper air becomes turbulent.
802:
341:. The name has since come to be widely used. There is a companion
314:
141:
86:
798:"Ask A San Francisco Native: Has The Fog Always Been Named Karl?"
745:"How Karl the Fog rolls: Twitter presence’s identity is unclear"
131:
607:"Fog over San Francisco thins by a third due to climate change"
64:
1005:
Fog Today: Live satellite view of San Francisco coast and bay
566:
Fog and Boundary Layer Clouds: Fog Visibility and Forecasting
522:, Vincent Schaefer and John Day, p.75, Houghton Mifflin, 1981
927:
Smith, Jaqueline (2001). "'Advection' and 'Advection Fog'".
1021:
328:
259:, and is concerning for the local ecology, for example the
950:
Holton, James R.; Judith A. Curry; John A. Pyle (2003).
327:
account for the San Francisco fog, inspired by the fake
345:
account. The name is also used throughout episode 8 of
458:"Water Temperature Table of the Central Pacific Coast"
305:
The Beautiful Unseen: Variations on Fog and Forgetting
1004:
384:
Johnstone, James; Dawson, Todd (February 16, 2010).
931:
Facts on File Dictionary of the Weather and Climate
928:
762:"National Weather Service kills off Karl the Fog"
331:public relations account that appeared after the
150:through the Golden Gate, seen here in August 2012
390:Proceedings of the National Academy of Sciences
319:Karl the Fog playing with the Salesforce Tower
295:made a film about San Francisco's fog for the
242:San Francisco panorama looking northeast from
59:extending south to the northwest coast of the
181:which also communicates via the Bay with the
8:
627:"The Elusive Future of San Francisco's Fog"
51:Fog is a common weather phenomenon in the
27:Common weather phenomenon in San Francisco
419:
409:
55:as well as along the entire coastline of
38:
555:, Harold Gilliam, UC Press, 1962, 2002.
553:Weather of the San Francisco Bay Region
376:
904:"Ship's pilot blamed for bridge crash"
592:, Western Geographic Science Center,
7:
952:Encyclopedia of Atmospheric Sciences
850:SFGATE, Eric Ting (August 5, 2020).
323:In 2010 an anonymous person began a
625:Branch, John (September 15, 2022).
462:National Oceanographic Data Center
25:
1054:Natural history of San Francisco
1009:
231:
126:, and if small particles called
107:Along the California coast, the
954:. Vol. II, Fog. New York:
713:The Los Angeles Review of Books
594:United States Geological Survey
520:A Field Guide to the Atmosphere
482:Nolte, Carl (August 19, 2005).
333:oil spill in the Gulf of Mexico
226:", "No Sky July" and "Fogust".
173:The largest coastal gap is the
104:near the surface of the ocean.
976:Cambridge Guide to the Weather
299:, which premiered in 2013. In
1:
902:Nolte, Carl (April 4, 2013).
532:James William Steele (1888).
138:Land/sea temperature gradient
676:"San Francisco's famous fog"
18:San Francisco fog generation
1028:Karl, the San Francisco Fog
935:. New York: Facts on File.
265:Pacific decadal oscillation
91:Fog outside the Golden Gate
1070:
1044:Geography of San Francisco
981:Cambridge University Press
707:Heather Scott Partington,
29:
665:, retrieved May 24, 2018.
596:, retrieved May 24, 2018.
538:. Rand, McNally. p.
61:Baja California Peninsula
1024:Official X (ex. Twitter)
444:Climate of San Francisco
117:Sea surface temperatures
973:Reynolds, Ross (2000).
838:San Francisco Chronicle
821:San Francisco Chronicle
766:San Francisco Chronicle
749:San Francisco Chronicle
680:San Francisco Chronicle
488:San Francisco Chronicle
411:10.1073/pnas.0915062107
267:is generally rejected.
158:, a large area of high
320:
151:
92:
53:San Francisco Bay Area
48:
318:
145:
90:
42:
1018:at Wikimedia Commons
1016:Fog in San Francisco
840:, February 21, 2016.
615:, February 15, 2010.
160:atmospheric pressure
30:For other uses, see
784:The Washington Post
768:, October 30, 2018.
715:, February 9, 2015.
402:2010PNAS..107.4533J
177:at the entrance to
128:condensation nuclei
63:. The frequency of
1030:Official Instagram
698:, October 1, 2013.
631:The New York Times
321:
311:In popular culture
285:August Kleinzahler
156:North Pacific High
152:
109:prevailing current
93:
49:
45:Golden Gate Bridge
1014:Media related to
751:, August 8, 2017.
694:, Press release,
663:Skeptical Science
605:Richard Alleyne,
396:(10): 4533–4538.
179:San Francisco Bay
148:San Francisco Bay
32:San Francisco Fog
16:(Redirected from
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743:Heather Knight,
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734:, June 13, 2013.
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724:Mike Billings,
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682:, July 6, 2009.
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251:Climate change
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164:Central Valley
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83:Ocean moisture
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77:climate change
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67:and low-lying
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297:Exploratorium
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261:redwood trees
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43:Fog over the
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1022:Karl the Fog
979:. New York:
975:
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911:. Retrieved
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885:. Retrieved
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859:. Retrieved
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709:"Fog Chaser"
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642:. Retrieved
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491:. Retrieved
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465:. Retrieved
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281:Jack Kerouac
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168:Coast Ranges
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106:
102:marine layer
94:
50:
36:
913:October 21,
887:October 21,
815:Amy Graff,
760:Amy Graff,
301:Kyle Boelte
175:Golden Gate
146:Fog enters
1038:Categories
644:October 3,
493:January 4,
467:January 4,
371:References
361:Bay Bridge
244:Twin Peaks
224:June Gloom
212:Turbulence
194:Variations
57:California
47:(May 2009)
861:August 5,
731:SF Weekly
639:0362-4331
355:Accidents
343:Instagram
293:Sam Green
277:Herb Caen
113:upwelling
430:20160112
365:Martinez
338:Big Fish
124:dewpoint
73:tule fog
421:2822705
398:Bibcode
325:Twitter
216:monsoon
987:
962:
939:
908:SFGATE
882:SFGATE
856:SFGate
637:
428:
418:
287:, and
803:SFist
985:ISBN
960:ISBN
937:ISBN
915:2022
889:2022
863:2020
646:2022
635:ISSN
495:2014
469:2014
426:PMID
132:kelp
95:The
1049:Fog
540:175
416:PMC
406:doi
394:107
65:fog
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20:)
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