FOG

PJ Croft, University of Louisiana at Monroe, Monroe,LA, USA

Copyright 2003 Elsevier Science Ltd. All Rights Reserved.

Introduction

Murky and perhaps even eerie, or pristine and serene,are just a few of the descriptions of the often blindingwhite–gray veil that comes tomindwhen people thinkof fog. Whether over oceans or local waters or overvarious landscapes, a certain uniformity and blank-ness is associated with fog. Often hugging the terrain,or simplymasking the landscape, fog compromises theintegrity of our senses that are honed to a fog-lessenvironment. Sometimes it is like an unbending solidwall, many times vanishing over short distances untilanother patch is encountered, and even the beauty of afogbow belies the significance of fog.

Merely a collection of small-diameter suspendedwater droplets in the air, fog may occur in a calmatmosphere that is saturated, or nearly so, and mayalso occur in cool and moist air moving quickly pastus. Although fog is commonly thought of as merely ‘acloud on the ground’, it is much more than this givenits very dynamics of formation, ‘intensity’ (or thick-ness), nature of the droplets that it consists of, andareal extent andduration. Fog occurs around the globefor many different reasons and can be elusive whenpredicting its exact occurrence.

Will fog form? Where will it form? When will itform? How ‘thick’ will it be? How long will it persist?These are only a few of the many questions forecastersand people ask about the enveloping droplets of fog.Yet to examine fog occurrence it is necessary toconsider first its impact on our activities and then learnmore about its characteristics and physical behavior.Beyond fog impacts, the observation and study of foghelps us to define its characteristics more completelyand thus aids in our understanding of the fog process.With this information we are able to better predict itsoccurrence, extent, intensity, and duration so as toavoid or mitigate some of the hazards associated withfog. In fact, such an examination provides us with anopportunity to make use of fog in various agricultural,military, andother applications.The significanceof fogand fog prediction includes impacts as well as benefits.

Impacts

Fog occurrence impacts a wide variety of humanactivities worldwide. These impacts range from in-

convenience to annoyance and from high costs todeadly consequences. Although mostly negative con-sequences and perceptions are associated with fog,there are positive benefits as well. These range from apleasing esthetic effect to fog harvesting for agricul-tural and water supply applications. Thus repercus-sions can lead to a variety of associated political,social, and legal implications, depending upon theprecise impact and the person or peoples – andeconomies – affected. The first and most obviousimpact of fog is related to the reduction of visibility.This reduction hampers and restricts our navigationalabilities and thus increases our chances of judgmenterrors in the operation of transportation vehicles.Reduced visibility in fog quickly impacts our ability todrive,move overwater, fly, and transit land by train. Ineach case the inability to seewell, or to see an adequatedistance ahead, is compromised by both fog and thespeed ofmotion. It is further compromised by our ownocular inability to distinguish objects given limitedbrightness and contrast that occur with fog.

Land Transportation

Land transportation includes automotives, trucks,and heavy machinery and is prone to disruption anddelay when fog is present. Near Windsor, ON (Can-ada), a highway ‘pileup’ collision during morning fogin September 1999 resulted in seven deaths as 62 carsand tractor-trailers collided. In the United States inKingsburg, CA inNovember – andWaynesboro, VA inApril – heavy fog resulted in highway pileups thatkilled 42 and injured 91 people as 40–65 vehiclescollided in mountain and valley regions in 1998. TheVirginia pileup is a ‘chain-reaction’ crash in a regionprone to ‘heavy’ fog, heavily traveled, and whichfrequently experiences low visibilities. A tour bus andtruck collided in Asuncion (Paraguay) in March 2000while traveling through early morning dense fog and30 of 45 people on board were killed. A caravan ofbuses transporting college students in Pennsylvania(United States) traveling through dense fog overnightcollidedwithone another killing twoand injuring 106.

In Bourg-Achard (France) in September 1997,several chain-reaction crashes claimed eight lives andinjured 63 as over 100 vehicles were involved during amid-morning ‘heavy’ fog event. Witnesses and victimsreported that visibility was merely 45 yd (41.148m)when the crashes occurred. On the Ivory Coast inAbidjan (Africa), ‘thick’ fog combined with dustywinds from the Sahara Desert during the HarmattanSeason in December 1995 killed 14 and injured 86.

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News reports indicated that a similar accident inAugust killed 20and injured 62, and that drivers in thisregion are known for speeding and for their reluctanceto diminish their speeds evenwhenweather conditionsare poor. In Lisbon (Portugal), four were killed and 70hurt in a 100 car pileup in January 2000 that halted upto 6000 cars in both directions of a 20mile roadwayfor 5 h.

In Mobile, AL in the United States, 193 vehiclescollided on the Mobile Bay ‘Bayway’ highway in thecountry’s worst fog accident ever in March 1995,sending 91 injured to hospitals and killing, miracu-lously, only one person. Insurance losses were esti-mated at over one million dollars at the time of theaccident, and witnesses and victims report sitting intheir cars and listening to the continuing crashesbehind them. Some report driving ‘into a wall’ of fogwith visibility immediately reduced from 0.5 mile(0.8045 km) to near zero. The roadway was closed forhours in both directions of travel. The event led to theinstallation of fog sensors in the hopes of avoiding arepeat of the accident. The same was done for a fogwarning system in Waynesboro at a cost of over fivemillion dollars.

Although many deaths are directly attributable tocollisions, a number are caused by fires ignited duringthe collision process.Many factors lead to such seriousconsequences during fog events. These include poorvisibility, vehicle speeds (posted as compared totraveled), traffic volume, roadway design and surfac-ing, driving habits (which include invincibility andtrust of braking systems, e.g., anti-lock systems),roadway conditions (perhaps dry but sometimes wetdue tomist or drizzle, previous rains, or condensation)that restrict braking ability, and windshield visibilityeffects. Although fog has been cited as the primarycause of an accident in generally less than 1% of allaccidents in a given region, it has been cited up to 10%of the time in a fog-prone region, particularly inmultiple collisions. The average claim for one vehicleis nearly $8000 (US) and over one million dollars for amultiple-vehicle crash.

The most obvious threat is reduced visibility whichrestricts a driver’s ability to navigate the roadway. Thisis further diminished with increasing speeds and ofserious consequence resulting in many deaths andinjuries every year around theworld. The visibility notonly restricts horizontal distance and depth percep-tions but also reduces the ability of drivers to gaugetheir own speed of travel. Through computer simula-tions a psychologist was able to determine thatalthough drivers could learn to sustain their speedsin simulation, the addition of fog distorted or de-stroyed this ability. Ironically, many drivers are pronenot to check their speeds in a fog situation as they seek

desperately to maintain their scanning of the roadwayahead to ensure their own safety.

In addition, the variation of fog intensity andduration create a rapidly changing set of visibilitiesduring the course of travel and may be furtherenhanced by hilly terrain and/or protected regions.Although no criteria exist for safe driving in fog, it isclear that visibilities under 1mile (or 1.6 km) whiledriving at speeds of near 60miles per hour (i.e., onemile per minute; or 96.54 kph) compromises seriouslya driver’s integrity and response time to hiddenhazards. This is often exacerbated by the distancebetween vehicles and curved or inclined sections ofroadway. Thus the first lines of defense for navigatingfog is the reduction of speed, the use of headlightsand/or flashers, fog lights or fog-free lenses andshields, and the ‘stop, rest, and wait’ approach. Otheralternatives include fog dispersion or mitigation tech-niques discussed later.

Rail transportation may be impacted similarly byfog conditions. In Badrshein (Egypt), on theNile Riverin December 1995 at about 8.00 a.m., one passengertrain plowed into an express coach which had sloweddue to ‘heavy’ fog. The wreck killed 75 and injured 76as five train cars were destroyed and 40 damaged.Reportedly the train’s driver could not see even a yard(approximately 0.9144m) to the front and hadapparently ‘stuck his head out of the window’ to tryto see better. Rail collisions have also involved motorvehicles and marine vessels at various crossings.

Marine Transportation

The operations of ships and barges, pleasure craft, andsailboatsmay all be hampered by fog. Fog events oftenslow, and may even stop, marine operations withsignificant economic costs. Awell-known event is thesinking of the Andrea Doria in July 1956 off the mid-Atlantic coast of the United States after collision withthe Swedish liner Stockholm. One of the most seriousincidents occurred in the northern portion of MobileBay, Alabama (United States) in September 1993.During the earlymorning hours a barge collidedwith arail span, moving it out of alignment moments beforethe arrival of an Amtrak passenger train. Rescueoperations were severely hampered due to inability toreach easily the crash location and survivors as 47people died. Three years later the country’s CoastGuard implemented rules requiring towboats to beequipped with radar, searchlights, radios, compasses,and other navigational gear and that the crews knewhow to use them.

Shipping operations are stopped or slowed whenvisibility is as low as 0.25mile (0.4023 km). In amajor(or minor) port or cargo region, large financial losses

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can occur when operations are halted. In the case ofdelays, reduced supplies and delivery of criticalelements and products limit the productivity ofindustry and commercial interests and their ability toprovide services to their clients. At the same time, shipoperations require daily operating and maintenancecosts, salaries, and living expenses which decreaseprofits. In the case of halting operations, theseconsiderations also include reduction or eliminationof the viability of product, particularly if perishablesor refrigerated goods are being transported. Lossesmay range from $10 000 to $25 000 per day per shipand into millions for even a moderately active harboror port of call.

Air Transportation

Aircraft impacted by fog include airplanes, blimps,balloons, and helicopters. Although fog-related crash-es have occurred for large aircraft in the past, this ismuch less frequent today given instrument flightregulations and improved navigational beacons andtechnology as part of air traffic control. Smaller planeshowever are more prone to difficulty as they lack suchsystems. The more typical impact of fog today is thedelay anddiversion of flights. InHongKong (China) inMarch 1998, 10 000 passengers were stranded at theairport as over 50 flights were canceled, delayed, ordiverted. Dense fog events in Minneapolis, MN(United States) in November 1997 and February2000, and at the Jackson Mississippi (United States)International airport in June 1996 resulted in flightdelays for thousands of travelers during the morninghours. At LaGuardia Airport in New York(United States) in February 1996, delays claimedmost travel plans for an entire day as visibility wasreduced to 300 ft (91.44m). Some passengers wereshuttled to nearby airports 10–30miles (16–48m)away to take alternate flights. Most flight operations,regardless of instrumentation, require at least0.75mile (1.207 km) visibility for takeoff andlanding.

Weather has been reported as responsible for one-fourth to one-half of all aviation accidents in theUnited States, including fatal accidents, with anaverage of over 400 lives lost each year. The delays,diversions, and cancelations resulting from fog add tothe cost ofmajor accidents. Delays and diversionsmayresult in greater costs due to fuel usage, passengerdiscomfort and complaints, and the shuttling ofpassengers to alternate flights or airlines. Cancelationsresult in displaced passengers and flight crews andcreate additional costs of lodging, food, and alternatetransportation. Depending on plane size and passen-ger loads, and regardless of an airport’s preponderance

for fog, a delayed or canceled flight may cost an airlinebetween $5000 and $25 000 per flight.

Military, Rescue Operations, and Other Impacts

Although these are the most common types of impacts(Figure 1), many other impacts do occur for a varietyof military and rescue operations and other activities.For example, theD-Day invasion and other theaters ofengagement have benefited or suffered from the effectsof fog. The deployment of troops in Tuzla Bosnia-Herzegovina by the United Nations was put ‘on-hold’for several days as persistent and continuous fogclaimed the land inDecember 1995. The rough terrainis known for its bad weather and thus allows onlysmall windows of opportunity for flight operations tobe made safely. In addition, rescue operations for acargo ship, which had collided with another off theSouth Korean coast, were suspended in June 1996 aspersistent fog limited visibility to less than 10 yd(9.144m). Space shuttle operations, as well as delayedlaunches and landings, have been impacted by fog orthe prediction of its occurrence.

Although rare, fog has caused sporting event can-celations (baseball and football) or suspensions andclass delays or cancelations at schools and colleges(especially if a large number of commuter students).Fog has also played a role in dangerous air pollutionepisodes including Belgium in 1930, Pennsylvania in1948, and London in 1952. More recent eventsinclude fog combining with forest and oil fires,chemical spills, and other emissions. These events aresometimes aided by topographic variations (e.g., avalley location, river valley, or other water source),regional climate (e.g., coastal), and nature of humanactivities (i.e., industrial or agricultural) in a stableatmosphere (e.g., Mexico, Arctic).

Air pollutants can also act as condensation nucleiand create lower visibilities through haze and fog. Thiswas recognized both in early Roman history as well asprior to that time. Some of the most intense fogs resultwhen high concentrations of pollutants and otheraerosols are found in the air. London fog and itscombination with pollutants, recognized as early asthe 1660s, was brought to the attention of KingCharles II and Parliament. In Donora, PA duringOctober 1948, nearly half the population of 14 000became ill during a prolonged valley air pollutionepisode that was accompanied at times by fog. Theenactment of air quality regulations has aided in theabatement, but not the elimination, of such impacts.

Legal Implications and Mitigation

Although no direct fog insurance exists, the costsassociated with fog events and disasters may be

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covered according to damage, repair, and replacementas well as personal suffering or loss. The liability maynecessarily carry high rates as a function of thenumbers of people involved and the nature of the‘cargo’ being transported, particularly in shipping, orthe nature of the impact, such as in an air pollutionepisode. Payment is made based on a judgment ofwhether the situation was avoidable, whether actionswere prudent in the particular setting, and if the eventwere predictable or an ‘act of God’ or a ‘once-in-a-lifetime’ event.

The sociopolitical decisions and mandates thatcome from these find their way into legislation orstandards of procedures as evidenced by aviationinstrument flight rules (IFRs), port regulations forclosure and/or delay/diversion, and the recommenda-tion (or requirement) that drivers use low-beamheadlights when fog is present. Legal implicationsmay arise for a farmer who irrigates a crop (for wateror for frost protection) near a roadway and thusenhances the fog and which results in an accident.Clearly, liability is one of the reasons for moreconservative decisions on closings, delays, and can-

celations. It will also include those who clear fog atairports and other locations if they do not meet theirobligations under contract and are taken to court fordamages.

Observations

The most obvious types of observations of fog are itsoccurrence, reduction of visibility and/or fog’s ‘thick-ness’ or ‘intensity’, color (of limited use in reporting),duration, and extent. Each of these may be assessedlocally at an observation site and regionally using acollection of sites. The traditional determination ofvisibility in fog is based on the ability of a humanobserver to see predetermined ground-based targets inall azimuth directions about their site. Electronicmethods (e.g., transmissometer) have been applied toroadways, harbors, and aerodrome runways to deter-mine low visibility in the immediate location of thesensor. Various methods and instrumentation used todetermine visibility are presented in the visibilityportion of this encyclopedia.

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Cold riverwater

AirportIndustry Rail

depot

Rowcrops

Driver visibility

Headlights

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RoadwayBrake lights

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• Surface conditions• Cloud microphysics

The figure shows varyingterrain and patches of

fogvarious intensities in a

'big picture' perspective.Airport/Rail Depot in

Valley Flat Land

(where written) with

Figure 1 Fog transportation factors. The figure shows varying terrain and patches of fog.

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The occurrence and reduction of visibility are easilyrecorded based on human observation, but are oftenelectronically derived through the use of transmisso-meters and other instruments (e.g., the ASOS used bythe National Oceanic and Atmospheric Administra-tion’s National Weather Service as well as airportsaround the world) and through remote sensing plat-forms. The duration and extent are consideredthrough such platforms as well. The remote sensingtools also include satellite, radar, and lidar. Satelliteobservations (both infrared and visible) can indicatefog location and extent and infer fog ‘thickness’ in amanner far superior to a collection of surface obser-vation sites across a region.Theuse of satellite imageryis very helpful in completing the depictionof fog extentand ‘intensity’ for a region. Visible imagery oftenshows a sharp-edged boundary between adull gray fogregion, whereas infrared imagery requires a tempera-ture differencing technique to indicate fog areas. Ineither case the ‘strength’ or signal ‘brightness’ shownby the image is related to its continuous nature (e.g.,patchy versus widespread) and thickness (strength ofsignal) of the fog area.

However, satellite observations have limitationsincluding minimal temperature contrasts on infraredimagery and intervening cloud layers on visibleimagery. In addition, satellite observations typicallyare not current as processing and dissemination ofimages may take an average of 10–45min. In otherwords, satellite imagery is of limited value in terms ofprediction of occurrence. Radar, although not adetector of fog droplets, is useful in identifyingvariations in the refractive index within a limitedradius of the radar site. This can be used to infer thepresence, or development, of an inversion and itsheight which may indicate potential fog development.The use of lidar is designed to detect even lowconcentrations of small particles and can be used toresolve fog formation and occurrence within a limiteddistance of an observation site.

Definitions

Fog consists of suspended droplets, some of whichmay be settling out and/or evaporating, which restrictvisibility and persist for some period of time. Fog mayvary in depth occurring within the lowest meter of theatmosphere and extending up to 1000m in height.Occasionally the fog layer may be somewhat elevatedabove the surface, particularly during the fog disper-sion, breakup, or ‘burnoff’ process. For condensation,the relative humidities of the air do not necessarilyneed to be 100% and in fact may be as low as 80–90%both during and after formation. These conditions aremostly associated with a temperature-dew point

spread of up to 51F (approximately 31C) and a stablelayer of atmosphere. Although fog occurrence isrelatively rare near or below the melting point ofwater, freezing or ice fog is possible. Other definitionsof fog relate the fog’s source and/or method offormation to its name.

Fog may form in place, be transported from onelocation to another, and may form in minutes or overan hour, depending upon existing conditions. The twobasic processes responsible for formation, as well asduration, are radiation and advection (which includesvertical mixing of air). It is based upon these observa-tions that fog is often referred to as ‘a cloud on theground’ and which consists of visible hydrometeors.However, the fog formation process has importantdifferences from clouds (including, for example, totalmoisture content, droplet size distributions, andchemical contents). By international definition, fogoccurs (or is significant) when visibility is restricted toless than 1 km (0.62 miles) and is distinguished fromthe occurrence of mist (or drizzle).

Fog droplets range in size because of the variouscondensation nuclei with which they form and ac-cording to their resultant wettability and solubility(and thus their physicochemical composition). The con-densation process is best illustrated through Kohlercurves that show droplet radius versus saturation level(i.e., relative humidity of below or near 100% throughsupersaturation of up to 0.5%). As the droplet radiusgrows, the saturation vapor pressure decreases andthe environment becomes supersaturated with respectto the droplet. This allows the maintenance ofdroplets in an equilibrium state. Because of theserelationships, maritime nuclei often have very smalldroplet radii and thus little need for supersaturationconditions.

Cited values for fog droplets range from 1 to 65 mmin diameter with an average diameter of 10–20 mmmost often reported (although some sources state therange as 20–40). Some observational studies differen-tiate between small droplet fogs (often maritime inorigin) and larger droplet fogs (more typically conti-nental). Maritime fogs are often observed to be morecontinuous and ‘thick’ due to their smaller dropletsthat allow a greater concentration within a parcel ofair. The occurrence of fog with light rain or lightdrizzle is not uncommon and typically limits the fog’s‘thickness’ and duration due to the fall of precipita-tion, mixing, and wind flow that accompany them.

People tend to characterize fog as thick or shallow,and some definition of this is found within the WMOcoded synoptic and METAR observations. Fog isusually distinguished from haze according to visibilityconsiderations, relative humidity, and content. Hazeand fogmay occur together andhave nearly equivalent

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impacts, but fog’s restriction of visibility typicallypredominates. Somewhat by convention, fog ‘thick-ness’ (or ‘intensity’) is defined according to therestriction of visibility. Dense fog produces visibilitiesof less than 1 km,moderate fog 1 to less than 5 km, andlight fog 5 to less than 11 km.These definitions rely onsight distance as a surrogate for measurement ofdroplet distributions and are exclusive of the occur-rence of low stratus clouds.

Fog duration and coverage, although observed intime and space, do not have a generally agreed upondefinition. Fog studies often differentiate betweenlong-lasting (i.e., several hours) and short-durationfogs (less than 2 h), and in some cases examineextended events (i.e., several days). Some studiesconsider a minimum one-degree ‘square’ of fog to besufficient to depict it as a regional occurrence, withsmaller areas being defined as local coverage events.Extended events lasting more than a couple of daysinclude late winter and springtime events in NewEngland and the mid-Atlantic states as well as winter-time events over the western US valleys, the Gulf

Coast, central Europe, and the Po Valley in northernItaly (Figure 2).

Climatologies

Global frequencies of fog are traditionally based onsurface observational data and consider the number ofdays on which fog was reported. Few climatologiesfocus on the time of day, extent, duration, or intensityof fog which would make for a useful basis ofworldwide comparison. Fog frequencies tend to behigh in locations where moisture is plentiful (oceanic,river/lake, and coastal regions as well as humid ortropical locations) or cooling processes predominate(mountain–valley locations and ocean currents). Asmost frequencies are derived from surface-basedobservations, they are necessarily skewed by popula-tion centers and established habitable regions and thusfog ‘hotspots’ are only approximations of the trueglobal occurrence and distribution. It must be notedthat even a region with a low fog frequency is notimmune to the devastating impacts of fog. Some

Airborneradar

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500 mModerate fog

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Regional fog event1° Lat./Long. area

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Duration≤2 hours short-term>2 hours long-term days extended event

Other characteristics• Chemistry and pollutants• Moisture content• Association with haze, drizzle, rain• Color• Inversion levelFog intensity

Dense <1 kmModerate 1 to <5 kmLight 5 to <11 km

Formation• Condensation • RH 80−100%• T/Td spread 0−3°C (0−5°F)• Droplet size distributions• Radiative and Advective processes

10 m

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Figure2 Observations and definitions. The figure gives a perspective view, as inFigure 1, with focus on airport location and equipment

and varying fog intensities with spatial scale shown and varying depth of fog layer.

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regions may experience an average of 100 daysper year with fog while others may only average lessthan 10.

Hemisphere has the majority of land-based andcoastal zone fog occurrences given the spread ofcontinents around the Earth. Many regions in NorthAmerica, Europe, India, Africa, and Asia witnessdebilitating fog events, some sporadic, others long-lasting. A variety of climate zones are affected,including polar, temperate, tropical and even dry tomoist. Locations with the highest frequencies arefound in the vicinity of cold ocean currents and/orupwelling that stabilizes the atmosphere, provideslocal moisture and aerosols, and can cool air quickly –and do so for a prolonged period of time – tocondensation. For these locations fog may persistfor several days, cover an extensive area, and thenmove en masse to another region. These fog regionsmay occur any time of day and tend to be morepersistent during the daytime with little diurnalfluctuations.

The SouthernHemisphere also has a variety of land-based fog in regions such as South America, Australia,South Africa, and the Antarctic. As strong winds andmixing predominate over much of the SouthernHemisphere oceans, fog regions tend to persist in thecoastal zones (e.g., Chile) and/or over the interiorwhere terrain varies considerably (e.g., Argentina). Inaddition, the tropical rain forest regions provide amplemoisture and sufficient radiational cooling overnightfor the formation of large fog areas. In these cases, thetime of year and time of day vary from the NorthernHemisphere. Further observations are much morelimited given smaller population bases and/or the lackof observations or observational equipment in theSouthern Hemisphere. A wide variety of remotesensing methods and tools have been used to refineand localize these values but have not as yet provided acomplete view. For example, aerosol climatologywork and cloud climatologies based on satellite andradar observations can more readily complete thepicture of fog occurrence, and thus frequencies andclimatology, for these parts of the world.

The importance of the foregoing discussion of fogclimatology is that it helps to identify fog-proneregions, episodic regions, and assist in the forecastingprocess. It also aids in the identification of common-alities of fog formation and thus offers clues as to theevolution and behavior of fog. Some fog regions tendto be synoptically enriched or dominated and othersmore by boundary layer processes. Each plays a role inthe extent, intensity, and duration of fog and helpsdeterminewhether fog frequencies are bi- or tri-modalin time and according to global circulation regimes(e.g., central North America and the west coast of

South America as related to El Nino and La Ninaphenomena).

Location

A closer look at the common features of fog-proneareas of the world provides a basic breakdown ofmaritimepolar climates,Mediterranean climates (e.g.,France), and mountain/valley climates (e.g., Chile).Although fog is alsomore likely to occur in the vicinityof water sources such as ocean currents and rivervalleys andwarmwater springs (e.g., in cold climates),it may also be found in desert climates (e.g., Africa andAustralia). Radiational influences are maximized inthese desert regions as well as in valley or plateauregions around the world (e.g., China and Mexico).When advective influences predominate (such asNewfoundland), various lifting and cooling processesare more important. In addition, the intermediaryzone between ocean and land also plays a significantrole in the extent, intensity, and duration of fog.

Various combinations of these lead many to classifyfogs anddevelop a list of fog types thatmay occur. Thisallows us to distinguish between the climatic predom-inance of fog in some regions versus its origins, andaccording to the prevailing synoptic flow in real time.Clearly, this does not imply that other places do notreceive fog or that it is unimportant. It does provide acontext inwhich the scale of fog occurrence and extentmay be considered. These include synoptic-scalefeatures (e.g., high pressure), mesoscale variations(e.g., in moisture distributions), and microscale (andat timesmesoscale) features that affect the occurrence,extent, intensity, and duration of fog (Figure 3).

Scale

The role of synoptic versus mesoscale and microscalevariations is best understood through examination ofa site-specific climatology. However, this is less prac-tical – and less informative – when the same principlesare applied to a collection of sites. Instead it is best toexamine the patterns and/or fluxes that are importantat each scale to explore the characteristics of fog.These are also of use in the identification of fogprecursors which improve our understanding andprediction of fog formation, extent, intensity, andduration.

Synoptic-scale features include high- and low-pres-sure areas (with/without rain or recent rainfall),locations ahead of a warm front, the warm sector,and behind a cold front. In the simplest case, highpressure and sufficient near-surface moisture com-bined with overnight cooling produce fog as radiativeprocesses dominate. In the case of low-pressuresystems and frontal regions, some advective and

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Kuroshio

AlaskaCurrent

N. Pacific Current

N. Equatorial Current

Equatorial Counter-Current

S. Equatorial Current

Antarctic Circumpolar Current (West Wind Drift)

CaliforniaCurrent Florida

Current Gulf Stream

CanariesCurrent

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Figure 3 Fog climatology. Map of the world showing continents, cold ocean currents, topographic relief and some of the common locations favoring fog occurrence.

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radiative processes will dominate. Fog occurrence ispossible, whether inclusive of precipitation-inducedand/or cooling effects. The surface character, overwhich these features pass, then acts to promote ormoderate the cooling and/or lift. These includeupslope flow, cold ground (or frozen or snow or icecover), and vegetative contributions (in terms ofadditional moisture in the local boundary layer).

Mesoscale variations modify the imposed synopticconditions and may grow with time to be moresignificant if synoptic flow becomes stagnant (e.g.,autumn) or blocked (e.g., spring) or simply ‘vanishes’(e.g., low latitudes). Mesoscale variations in physiog-raphy and weather conditions are obvious in areassuch as San Francisco, Death Valley, Salt Lake City,and the Pacific Northwest (in the United States, WestCoast) and make a great deal of difference inforecasting the location, duration, and intensity offog. These variations are further complicated by thecomplexity of possibilities on the microscale whereboundary layer processes dominate. These tend to becritical in identifying the precise locationof formation,timing, intensity, and duration. It is also critical withregard to the microphysical aspects that involveaerosols. This immediately distinguishes betweenmaritime and continental concentrations and typesof fog droplets as previously described.

Formation Mechanisms

Fog formation requires a variety of factors in differentcombinations. Essential to fog formation are sufficientmoisture and the process of cooling and/or lifting(inclusive of mixing). This gives us three basic ‘types’of fog: radiational (cooling), advective (cooling and/orlifting), and combinatorial (cooling and lifting, ormixing). These may occur in several ways from thesynoptic to the local scale, as has been illustrated bythe different climates around the world which expe-rience fog. The more critical factor in the atmosphereis the presence of sufficient moisture, in terms of totalamount and depth, and its horizontal distribution.Sufficient moisture may also be achieved by increasingits ‘effectiveness’, in other words, making use of themoisture present and realizing it through coolingand/or lifting processes to cause condensation. In eachcase, despite synoptic influences, moisture and itsrealization is very much a function of mesoscale andmicroscale conditions and variations.

In addition to moisture and cooling/lifting mecha-nisms, fogs are observed to be associated with aninherently stable atmosphere. This stability mayprecede or occur after fog formation and oftenincreases with the advent of fog. Even fog that isassociated with strong winds, as is the case with some

advective fogs, occurs in relatively stable layers of theboundary layer. Other relevant factors in fog forma-tion which may be considered as secondary in nature(yet significant in the prediction of fog) include cloudmicrophysics, the vertical and horizontal distributionof temperature and moisture, orographic effects,sources and sinks of moisture and heat, and land useand/or surface conditions (Figure 4).

Cooling

There are several means of cooling an air mass, orparcel of air, that may lead to fog formation. Themostobvious and most prevalent (even in the presence ofcloud cover) is the diurnal loss of heat by the Earth’ssurface (i.e., radiational cooling).Othermeans includethe cooling of an air mass from below, adiabaticcooling (or mixing), the cooling of an air mass itselfdue to radiational release, and the evaporationalcooling of air due to precipitation through a dry airlayer whichmay induce cooling to saturation and thusresult in fog. Depending on the location, time of year,and moisture availability, these cooling mechanismsmay lead to fog formation with varying persistenceand of varying extent and intensity.

Radiational cooling is primarily diurnal in natureand is maximized overnight and during the earlymorning hours, with minimum air and surface tem-peratures often occurring at or near sunrise. Althoughthe diurnal cooling process occurs year-round, it isfavored during both dry and cold seasons of the yearwhen low-level moisture may be sufficient, relativelyundisturbed, and the cooling period lengthy. Thedryness of the atmosphere is most typically observedabove the boundary layer and allows great radiationallosses through an open atmospheric window, even inthe presence of middle- or high-level clouds. Radia-tional fogs may be brief in duration (e.g., less than 1 h)or may last several hours. The depth and intensity ofthese fog events is a function of the cooling time,extent, and amount of moisture available. It is notunusual for such fogs to initiate dew deposition.

The other means of cooling are of varying impor-tance to fog formation and duration. For example, thecooling of an air mass from below is favored inlocations and seasons in which the active surface layeris frozen and/or snow covered orwhen it experiences agreater albedo (e.g., fallow versus the vegetativegrowing season). Such fogs may form and persist forhours or days at a time and cover a relatively large areawith significant intensity. Adiabatic cooling of air isrelated to liftingmechanisms and therefore consideredin the next section. The cooling of an airmass itself dueto radiational release is typically a very slow processand likely to be an important factor for persistent fogs

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(e.g., sea fogs). The extent may be great but theinternal variations in coverage and intensity are largegiven the interactions between the air mass and theunderlying surface features. Evaporational coolingcaused by showery precipitation falling through alayer of dry air may be sufficient to lead to saturationand fog, but is typically of short duration and oflimited intensity. In the case of a synoptic-scale warmfront, such fog may form and persist for several daysand become quite extensive and intense with minimallocal variations.

Lifting

The second basic means of cooling air to achieve orsustain fog formation, or for realizing the effectivenessof the moisture present in a parcel of air, is throughvarious lifting mechanisms. These include orographiclift, frontal lift, adiabatic ascent, and mixing. In manycases, these processes involve advection and thus giverise to advective fog formation and transport. Al-though this implies that there are many lifting situa-tions in which fog may form, it is clear that most of

these situations involve slow vertical lifting over largehorizontal distances or the relatively slow and shallowvertical mixing of two distinct air masses in theboundary layer. In the former case, long-lasting,extensive, and intense fogs may be expected whereasin the latter short term, shallow, and patchy fog ofvarying intensity occur.

Slow vertical lift due to an upslope wind flow,parallel with the elevation gradient, will result indiscrete levels of cooling and saturation with increas-ing distance and transport. Although this process maybe slow in the initial formation of fog depending uponthe amount of moisture available in the air mass (e.g.,several hours to nearly 24 h), it is a resilient processthat can produce extended events of widespread densefog (i.e., up to several days are possible). Similarly,frontal liftingmayproduce similar conditions andmaypersist for some time dependent upon the rapidity ofchanges in synoptic features. Frontal lift is morecommonly warm in nature but may involve coldfrontal surfaces which are of lesser slope than a typicalcold front. In both orographic and frontal cases, theformation, duration, extent, and intensity of fog

Cooling

Mixing

Cooling

Mixing

Lifting

Cooling

Other factors• Wind• Stability• Horizontal/vertical temperature

and moisture• Synoptic features• Orographic features• Source/sink moisture and heat• Land use• Surface conditions• Cloud microphysics

Figure 4 Fog formation mechanisms.

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events is also a function of the underlying surface andits interactionwith the lifted air. For example, the flowof warm and moist air across frozen or snow-coveredground – or simply upslope – increases the depth,intensity, extent, and duration of fog.

This last process is an important aspect and illus-trates how two diverse air masses, initially unsaturat-ed, may mix to form a saturated air mass. The use ofsaturation vapor pressure curves can be made tocompare air mass properties as a function of theirvapor pressures versus the absolute saturation vaporpressure for various temperatures and pressures.When a cool, and relatively lower vapor pressure, airmass combines with a warm air mass with highervapor pressure, their mixing results in saturation. Thismay be seen by plotting the original vapor pressures atthe individual air mass temperatures and connectingthe two points with a straight line. When the linecrosses the saturation vapor pressure curve, the twomixed air masses will form a saturated air mass. Themanner inwhich these two airmasses combinemay bethrough isobaric mixing or weak adiabatic mixing.

Lift that involves the adiabatic ascent andmixing ofair is greatly dependent upon the existing boundarylayer which evolves during the mixing process toproduce fog. Although of limited extent, turbulentmixing through adiabatic ascent can result in fogformation which is typically of very short duration(i.e., less than a few hours), limited depth (e.g., groundfog), and highly variable in coverage and intensity.Such fogs may occur preceding and following thepassage of weak cold fronts with limited pressure andair mass differences, and often following the passageof scattered showers or light rain, and take place in aconditionally stable boundary layer. These fogs tend tobe infrequent and of short duration as the dynamicsare more likely to lead to low cloud (and ceiling)formation with drizzle. However, in some cases theymay persist and thicken over regions as the frontalboundary decays and/or becomes stationary. Theprocesses of adiabatic ascent and mixing also play arole in the formation of Arctic Sea Smoke and otherfogs in which the heat flux is rapid and results from atemperature differential rather than a period ofradiational cooling.

Lifting and Cooling

Based on the preceding discussion, it is clear that thereare many possible combinations which may producefog. It is therefore understandable why so many ‘fogtypes’ occur in the literature and are studied aroundthe world. For the same reason, it is clear that thesepossibilities raise the question ofwhether fog is readilypredicted and whether one type is readily identified

over another or whether one type may evolve intoanother. Since the orographic and frontal liftingprocesses are typically a gradual cooling process overlong distances whereas the radiational cooling processis gradual over time and specific to a location, it isreasonable to consider various combination fogs in thesame manner. It is also reasonable to incorporate theeffects of cloud microphysics, the vertical and hori-zontal distribution of temperature and moisture,sources and sinks of moisture and heat, and land useand/or surface conditions.

For example, given the features described above, thelongest-lasting, most intense, deepest, and potentiallymost widespread fogs may occur near a coastal regionwith a moist onshore flow in the vicinity of a warmfrontal (or topographic) or quasi-stationary boundary.This would be further enhanced or favored if the flowofmoisturewere sustained, the ground frozenor snow-covered (and thus the source of cooling maintained),and it was the cool season time of year. The formationand advection of sea fog tends to meet these criteria tovarying degrees around the world and create some ofthe foggiest regions known.Regardlessof origin, that iswhether the sea fog formed first through radiationalcooling or other cooling and lifting processes, it is clearthat a variety of factors produce and sustain fog. This isverified by observation of the movement of fog areasand their passage from water to land.

Microphysical and Other Aspects

Although the identification of cooling and liftingmechanisms is significant with regard to the conden-sation process for fog formation, alone they areinsufficient if not considered with regard to thenucleation process. A knowledge of drop size distri-butions, condensation nuclei associated with fogdroplets, local nuclei populations, and the resultingatmospheric chemistry are significant with regard tothe occurrence, extent, intensity, and duration of fogevents. Depending upon the population of condensa-tion nuclei, the initiation of fog droplet formation, andthe actual drop size distribution, fog developmentmayoccur within 5–15% of the saturation value of an airmass (haze is typically within 35%). The role of nucleiis determined by ‘how active’ they are in encouragingor discouraging the process of condensation (i.e., howhygroscopic or hydrophobic). The presence of con-densation nuclei may be local in origin, advected, orthe result of both processes and is significant when fogis considered in combination with smoke and pollut-ants. The ‘proper’ combination of nuclei can lead tolong-lasting and devastating fog events.

For example, there are a number of well-knowncases in which fog combined with, and its formation

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was aided by, industrial emissions. These createdunhealthy and dangerous air quality and low visibili-ties over several hours (and even days), causing death,illness, injury, and accidental losses. These can befurther modified according to the location in whichthey occur. For example, marine environments typi-cally produce a large number of small dropletswhereas continental locations are characterized bylarge droplets. In marine environments then it is clearthat while haze is favored in the daytime (and saltnuclei) and a high moisture source with mixing, fogmay still occur if these conditions are overcome (e.g.,at night by cooling and with the introduction ofsmaller droplets).

Regardless of the limitations to droplet size by theconcentrations of nuclei, it is the actual dropletconcentration that determines a fog’s opacity – andthat is often referred to as a fog’s intensity or thickness(or severity). Because of this the lowest visibilities infog events are associated with high concentrations ofsmall droplets. Thus again a wide variety of fogformations are possible, particularly when consider-ing the cooling and/or lifting processes and whenconsidering the location, transport, and interactions ofvarious nuclei across a coastal zone.

The ‘Family of Fogs’

The foregoing discussion thus provides a ‘family offogs’ in terms of formation, extent, duration, andintensity which may be enveloped in a conceptualmodel of fog dynamics. Although similar inmanner tothe conceptualizations of air mass thunderstorm tosupercell, or mesocyclone to wave cyclone, or variouslevels of sea/land-breeze model conceptualizations,the family of fogs is, at the moment, quite elusive.Although generous amounts of research have beenaccomplished and numerous modeling studies com-pleted to reveal more explicitly the cooling and liftingprocesses that may produce fog, they are as yetincomplete. They do not incorporate adequately theatmospheric chemistry and associated microphysicsthat are significant to the fog process and are poorlyunderstood and not routinely observed. There is also aneed to consider the interactions and interface be-tween the underlying surface overwhich fog forms andthese microphysical aspects of chemical and physicalbehaviors.

Fog Dynamics

Given the basic knowledge of fog formation mecha-nisms (or occurrence), and some knowledge of micro-physical aspects, it is possible to explore the dynamicprocesses involved and how they relate to fog intensity

(or thickness), extent (or coverage), and duration.These are predominantly radiative processes as liftingmechanisms are essentially cooling processes as well.It is assumed that if the proper combination of factorsexists, and that moisture is available in sufficientquantity and/or its effectiveness can be realized, fogformation is possible. Adequate moisture is providedthrough local evaporative fluxes, advective delivery, orevaporation of falling precipitation. Ideally these areall quantified operationally (whether modeled, ob-served, or forecast) tomake a precise determination offog occurrence, intensity, extent, and duration.

Radiative Considerations – Formationand Growth

Rapid cooling, but with limited mixing, is bestaccomplished through radiative heat loss by thesurface and by an air mass that is predominantly staticin nature and predisposed to stability. These condi-tions are favorable to the potential for fog formation,assuming sufficient cooling occurs and sufficientmoisture is available (or its effectiveness realized).Commonly, such conditions are associated with clear(and sometimes dry) air, light winds, and subsidence.However, radiative cooling does occur in the presenceof cloud cover andmay be enhanced or reduced by theactive surface overwhich air is present. Each of these ismaximizedwithin high-pressure areaswhenwinds arenear calm, the boundary layer is moist, and the mid-and upper layers of the atmosphere are very dry withstrong subsidence. These conditions can producewidespread, intense, and long-lasting fog. Less opti-mal conditions (e.g., those present in the midst of aweak low-pressure centerwith little pressure gradient)may also produce fog that is of limited extent,intensity, and duration.

Although radiative and advective process has beeninitiated, the lowest portion of the boundary layernearest the ground becomes the coolest and thuscreates amicroscale inversion. This inversion grows intandem with the rate of cooling and the net coolingover time and, as the air reaches saturation, may leadto fog formation. As the cooling process continues, theinversion layer grows deeper and fog may grow ordevelop upwards with time to several meters within afewhours. At this point the presence of fog itself beginsto feed back into the radiative balance aswarmth fromthe surface may be absorbed by the droplets and therate of cooling slowed. In addition, the upper portionof the forming fog and the fog layer itself continue tocool, thus strengthening the inversion, while somedeposition and/or reevaporationof fog droplets occursnearer the surface. Often at this stage the radiative

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processes immediately at the ground have slowed,become less important, and reach a temporary equi-librium inwhich temperature andmoisture conditionsremain nearly constant.

Although these processes dominate in a generalsense, they are complicated by the nature of the surfaceover which air lies (e.g., soil type), land use and cover,heat and moisture sources, and vehicular and othertraffic that create local turbulence. It is the combina-tion of these factors that dictates the areal extent andinitial intensity of fog and that accounts for variationsas the fog persists. Sand and clay soils radiate atdifferent intensities and thus can contribute to fogformation at different rates. In the simplest case, theground surface may be conducting heat from subsur-face layers and thus eroding fog formation from thebottom after its initiation. The surface may also beconsidered active if wet or dry, vegetated or barren,paved or natural, and frozen or snow-covered. In thecase of a wet ground, more moisture and enhancedradiative cooling are possible. For a frozen surfacethere is a strong limitation on the radiative cooling ofthe ground and a strong enhancement of the cooling ofthe air itself. In the case of an asphalt or concreteroadway, the radiative rates may favor more rapid fogformation whereas vehicular flow (and turbulence)may discourage or disperse it.

Duration and Extent

As the radiative cooling persists, the fog layer maygrow vertically and horizontally with time. In partic-ular, it is not unusual for the fog on the surface todiminish with time, through deposition (dew or frost)and weak induced convective mixing, giving theappearance of a lifting fog. In this process, themoisture content and subsequently the dew point aredecreased within the lowest portion of the fog layerandmay allow for further radiative cooling and fog re-formation. Thus some fogs appear to vary in theiroccurrence, depth, and intensity with time. Moreimportantly, the middle and upper portions of the foglayer are now the most ‘active’ in terms of theirbehavior. In particular, these layers continue to cool,resulting in an upward expanse of the fog, and thus theinversion layer, and therefore become somewhatindependent of the surface over which they lie.

The horizontal formation and spread of the fog,initially a function of the radiative properties of thesurface over which the air is found and which accountfor the often patchy nature of fog formation, nowbecomes a function of weak circulations or turbulenceabove the ground. In the presence of high pressure andlight winds, this may create a fairly uniform fog interms of intensity and duration. For a weak low-

pressure system, or perhaps in advance of a warmfrontal boundary, both in the presence of light winds,thismay create large variations in fog extent, intensity,and durations with frequent and rapid variations.These conditions also imply slow transport of a foglayer andmodification as the fog travels across varyingterrain and surfaces and as it encounters variations incondensation nuclei. Modifications include othercooling processes, such as orographic lift, which mayreduce or enhance fog formation, maintenance, andgrowth. For example, a fog that develops in thevicinity of weak low pressure may move upslope inresponse to a weak pressure gradient and eitherprecipitate out or intensify and deepen. The same fogmay encounter an industrial area with a variety ofnuclei, which may lead to changes in the drop sizedistributions and either accelerate or defer fog forma-tion.

Intensity

Once fog has formed, persisted, and grown, theintensity or thickness of the fog is of greater practicalsignificance. Fog intensity is a function of the concen-tration of small and large drops in terms of their abilityto reduce visibility to less than 1 km. When fog is inplace, there is a certain amount of droplet settling andseparation with time while cooling and condensationoccur at fog top and dissipation and deposition at fogbottom (in many cases). While this process is effectivein maintaining fog, it is also effective in stratifying thefog layer and generating variations in intensity. Inthese situations the middle and upper fog layersbecome the most opaque and thus the most difficultto navigate in terms of transportation. These effectsmay be negated to some extent given a greater degreeof turbulent mixing or the presence or introduction ofwind flow (and thus entrainment) near the top of thefog layer.

In addition, the characteristics of condensationnuclei, can lead to varying intensities. For example, avariety of pollution-enhanced fogs owe their extreme-ly low visibilities to specific chemical species. Theseencouraged fog formation prior to saturation, reducedthe amount of deposition, and created amore uniformdrop size distribution often characterized by its coloror smell.

Dissipation

The dissipation of fog, is a function of the processesthat act against cooling and condensational effectspreviously discussed. Therefore dissipation may beconsidered in terms of the ‘prevention’ of the processesthat favor formation and growth, duration and extent,and intensity. Many of these processes may reduce or

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eliminate fog within less than 1 h without contraven-tion, but more typically require several hours toovercome the inertial presence of the fog layer aswell as any underlying or continuing fog formationprocesses. Usually visibility improvements occurwithin the first hour or two as dissipation processesbecome dominant. Fog dissipation is typically longerin valley and coastal regions and during the coolseason and over cold waters – and in some cases maynot occur.

The effects of cooling are mitigated or overcomethrough direct solar heating of the ground surface (orpotentially the heating of fog droplets and the air layerin which the fog is found, but this is of minimalsignificance) and the destruction of a stratified orinversion layer through turbulentmixing. Thuswarm-ing and mixing are vital to dissipate fog and may beaccomplished through a variety of frontal or convec-tive processes. This dissipationwill bemost effective atthe bottom and top of the fog layer and around theedges of the fog area where the air is not nearsaturation. Thus fog is said to ‘burn off’ or ‘lift’ and‘shrink’ with time. The pace of dissipation, which willbe greater under an imposed pressure gradient andduring the warm season, may last a few hours.

Prediction

The fog process is quite dynamic and requires a greatdeal of physical knowledge and observation to beunderstood completely and predicted successfully. Inpractice however, this is difficult given the limitednature of our present understandingof fog and the lackof real-time observation, quantification, andmodelingof the chemical and microphysical behaviors that areinvolved. This is made more difficult by the lack ofprecise observation and modeling of the detailedsurface characteristics, the distribution of moisture,temperature, and their interaction. However, if theconceptual model of a family of fogs based on theprinciples previously described is applied, the chancesfor improved fog prediction may be increased. Ulti-mately even a microscale observation network wouldnot be dense enough to provide essential details toimprove prediction. Instead the further applicationand refinement of remote sensing tools and numericalmodeling will be necessary to better forecast fogoccurrence, extent, intensity, and duration in a widevariety of circumstances.

Current forecast practices for fog include climato-logical approaches, numerical guidance, observation-al methods, modeling, and other methods (e.g.,statistical or decision-tree methods and ArtificialIntelligence – or Expert Systems). Common to each

of these is the recognition of those processes andfactors important to fog formation including cooling,lifting, and mixing (cooling and lifting) mechanisms;surface and air mass moisture and characteristics; andthe chemical and physical behavior associated withfog droplets. Some of these are summarized by manyauthors and researchers according to a list of factorssuch as the prior existence of fog, precipitation areas,soil moisture and cover, temperature and stability,boundary layer variations, orographic contributions,the synoptic setting and flow regime, vertical andhorizontal wind flows, cloud cover, and advection.Regardless of the list, it is essential to consider thesynoptic-dynamic regime as it determines the charac-ter of the boundary layer – and how that changes withtime – to predict adequately fog occurrence, extent,intensity, and duration.

Climatological

One traditional means of fog prediction is based onclimatological considerations. This approach, al-though location specific, is a reasonable first approx-imation in identifying and summarizing the factorswhich produce fog in a given region. Typical climatol-ogies assess fog frequencies according to the time ofday and year (or by season), intensity, duration, andsometimes according to fog occurrence at multiplesites. Unfortunately, the climatological approachtends to oversimplify fog prediction by categorizingevents as fog ‘types’ with little regard to fog evolution.In addition, the results are biased for the site at whichthe climatology is based and can thus lead to predic-tions of no fog for a region which may fail.

The use of conditional climatology adds some valuebeyond simple climatology in that persistence andcontributing factors associatedwith a fog event can beanticipated. Yet this approach is also limited in that itdoes not address fog dynamics adequately and reliesheavily upon the data available. In total a forecastermay be able to assimilate climatological informationand estimate local variations with some experiencefor a region, but the process is pragmatic ratherthan scientific and is only slightly more skillful.Climatological predictions have only limited antici-patory value: knowing the types of situations thatfavor fog.

Numerical Guidance

Another approach to fog forecasting is through the useof numerical guidance, both raw and processed. Rawdata from an operational model may be plotted orexamined across a region and yield specific informa-tion with regard to moisture and cooling with regardto fog formation.However,model output has a variety

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of errors, is often available in only limited time andspace resolution, and typically does not adequatelydepict the boundary layer structure or behavior.Whenthe raw data are processed to produce graphical anddiagnostic analyses, surface and boundary layer plots,or to generate statistical forecast guidance, it is ofmuch greater value. The graphical diagnosis of thelocal environment according to model output is usefulin better identifying regions potentially favored for fogdevelopment (e.g.,moisturemaxima, cool air pockets)and thus can aid a forecaster in refining a prediction ofoccurrence and extent. This provides a mesoscaleprediction which can be partially verified throughsurface observations and satellite imagery.

When used to generate statistical guidance, modeloutput helps a forecaster assess the confidence level offog formation and extent and allows some speculationas to its intensity and duration. This guidance makesuse of both current observations and model predic-tions. For operational models with output statisticsfrom12 to 60 h into the future, there is some predictivevalue and skill improvement over climatology in theanticipationof fog.However,most statistical guidanceused in this manner is derived from regressionrelations that show current observations (or simplypersistence) to be the most important factor in fogprediction. Thus numerical guidance is of best value inanticipating fog during the first several hours, oranticipating the synoptic setting conducive to fogseveral days in advance.

Observational

The observational approach to fog prediction isextremely limited in an anticipatory sense, but tendsto have the greatest skill in terms of fog extent,intensity, duration, and ultimate dissipation. Thismeans that a forecaster has synthesized the prevailingsynoptic environment and its interactions with theboundary layer to understand why fog is present (orexpected) and why it will continue to persist ordissipate. Essentially a forecaster in this situation hasdeveloped and applied a conceptual model frame-work, based on theory, knowledge, and experiencewhich provides specification that no other currentmethod can match. The observational approachinvolves a wide synthesis of sensible surface weatherelements, middle and upper atmospheric data andanalyses, and a review of satellite (and perhaps evenradar) images. Practical application of radiative lawsand parcel theory through the use of soundingdiagrams (e.g., a skew-T chart, and even isentropicanalysis) allow a forecaster to ‘run’ a conceptualmodel of fog and to make a successful prediction.

Modeling

Many modeling efforts have been made to simulate aswell as to produce a fog environment and fog droplets.Presently, no operational model is capable of com-pletely generating these in real time for predictive use.Modeling has more recently focused on drop sizedistributions, deposition, and re-creation of the fogenvironment as well as the physical representation offog and the fog process. Limited studies, includingstatistical prediction, have focused on the prediction ofthe visibility restrictions of fog. The First InternationalConference on Fog and Fog Collection is indicative ofthe new and renewed importance of fog study.

Much work is now focused on collection tech-niques, particularly with regard to fog chemistry andits impacts with regard to vegetation. In this regard,fog modeling has served as an extension of agro-meteorological and climatological study as well asmicrometeorology and cloud physics. Some profes-sional consultants and companies now offer fogforecast products, and even assessments, many ofwhich focus on predicting the occurrence and intensityof fog for a specified location. A variety of othermethods, including statistical or decision-tree meth-ods and Artificial Intelligence (or Expert Systems),have found usefulness in the study of fog.

Benefits, Mitigation, and SpecialCases

Although a variety of severe impacts have occurredwith fog events around the world, efforts to mitigatethese impacts have been of limited success in part dueto the nature of human response (e.g., driving moreslowly through fog). However, fog has also yieldedsome beneficial effects in terms of esthetics andagricultural application. Fog has also been the subjectof literature and film and often accompanies thedepiction of paranormal and Halloween activities.Fog systems are used routinely to create fog and otherspecial effects by the film industry. There has even beena survey that found 28% of people engaged in sexualrelations more than once when it was foggy ascompared with only 11%who did when it was sunny.

Several facets of fog illustrate how its significanceextends beyond a simple reduction in visibility. Forexample, fog water use for deposition collection iscritical to agricultural operations in Chile, Mexico,and other regions of the world. Fog water may be ofuse in providing a potable water supply for others.Most often fog water is collected for agriculturalapplications and has been studied with regard to itseffects on the growth of giant Redwood trees inCalifornia (USA). Study also indicates that fog may

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play a significant role in the physical interactionsfound within plant canopies and their physiologicaland growth conditions. These are also related to fogwater pH and the production of acid fogs. These andother issues are being explored through various studiesaround the world.

There also exist programs and methods for thedispersion, or enhancement, of fog – particularly atairport locations – and the reduction of acid fogs. Thebasic methods tend to focus on heating of the fog layer(to evaporate droplets), downwash mixing (to entraindrier air), hygroscopic treatment (e.g., ice seeding) toprecipitate out, and the use of fog breaks (passivecontrol) to prevent formation or movement into anarea. The most effective methods tend to be those thatmatch the natural dissipative factors (i.e., mixing andevaporation) and that promote improved visibilitywithin an hour. Unfortunately most techniques arecostly, labor intensive, esthetically intrusive, and mustbe maintained until natural processes are capable ofcontinuing dissipation (often for at least severalhours). These measures are also impractical forroadways and therefore drivers instead rely on vehiclefog lights and fog-free lenses or shields. The enhance-ment of fog, although seemingly undesirable, is anindustry with commercial application for party sup-pliers and movie studios.

See also

Agricultural Meteorology and Climatology. Air–SeaInteraction: Freshwater Flux; Gas Exchange; Momen-tum, Heat and Vapor Fluxes; Sea Surface Temperature;Storm Surges; Surface Waves. Anticyclones. ArcticHaze. Aviation Weather Hazards. Boundary Layers:Coherent Structures; Complex Terrain; Convective Bound-ary Layer; Modeling and Parameterization; Neutrally Strat-ified Boundary Layer; Observational Techniques In Situ;Observational Techniques–remote; Ocean Mixed Layer;Overview;StablyStratifiedBoundary Layer; Surface Layer.Chemistry of the Atmosphere: Chemical Kinetics; GasPhase Reactions; Ion Chemistry; Laboratory Studies;Principles of Chemical Change.CloudChemistry.CloudMicrophysics. Coastal Meteorology. Deposition. Di-urnal Cycle. Humidity Variables. Hydrology: Groundand Surface Water; Modeling and Prediction; Overview.IsentropicAnalysis.Land–Atmosphere Interactions:Canopy Processes; Overview; Trace Gas Exchange.Lidar: Atmospheric Sounding Introduction; Backscatter;DIAL; Doppler; Raman; Resonance. Mesoscale Mete-orology: Overview. Microclimate. Numerical Models:Chemistry Models; Methods. Observation Platforms:

Balloons; Buoys; Kites; Rockets. Observations forChemistry (In Situ): Chemiluminescent Techniques;Gas Chromatography; Ozone Sondes; Particles; Reso-nanceFluorescence;WaterVaporSondes.Observationsfor Chemistry (Remote Sensing): IR/FIR; Lidar; Micro-wave.Operational Meteorology.Orographic Effects:Lee Cyclogenesis; Mountain Waves and StratosphericChemistry. Parameterization of Physical Processes:Clouds. Radar: Incoherent Scatter Radar; MST and STRadars and Wind Profilers; Precipitation Radar; SyntheticAperture Radar (Land Surface Applications). RadiativeTransfer: Absorption and Thermal Emission; Cloud-ra-diative Processes; Non-local Thermodynamic Equilibrium;Scattering. Satellite Remote Sensing: Aerosol Meas-urements. Static Stability. Synoptic Meteorology:Forecasting; Weather Maps. Thermodynamics: Moist(Unsaturated) Air; Saturated Adiabatic Processes. Turbu-lence and Mixing. Turbulent Diffusion. WeatherModification: Inadvertant. Weather Prediction: Adap-tiveObservations;DataAssimilation; EnsemblePrediction;Regional Prediction Models; Seasonal and InterannualWeather Prediction; Severe Weather Forecasting. WindChill.

Further Reading

Anderson JR (1985) Economic impacts. In: Houghton DD(ed.) Handbook of Applied Meteorology. New York:Wiley.

Air Weather Service (1979) General Aspects of Fog andStratus Forecasting. USAF AWS TR 239.

Croft PJ, Pfost R, Medlin J, and Johnson G (1997) Fogforecasting for the Southern Region: a conceptual modelapproach. Weather Forecasting 12: 545–556.

Eagleman JR (1991) Air Pollution Meteorology. TrimediaPublishing Company.

George JJ (1960)Weather Forecasting for Aeronautics. NewYork: Academic Press.

Houze RA Jr (1993)CloudDynamics. NewYork: AcademicPress.

Mason J (1982) Physics of radiation fog. Journal of theMeteorological Society of Japan 60: 486–499.

Mayer WD and Rao GV (1999) Radiation fog predictionusing a simple numerical model. Pure and AppliedGeophysics, in press.

Schemenauer RS and Bridgman H (ed.) (1998) Proceedingsof the First International Conference on Fog andFog Collection, 19–24 July 1998, Vancouver, BritishColumbia, Canada.

Online reference sources

http://meted.ucar.edu (COMETModule Radiation Fog)http://www.cco.net/Btrufax/fluoride/fog.html

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