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Types of FogTypes of Fog
A brief by Lance GrindleyA brief by Lance Grindley

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3. © Grunt Productions 2008
• Radiation fogRadiation fog generally forms when the air near the surface cools togenerally forms when the air near the surface cools to
its saturation temperature due to radiational cooling at night when theits saturation temperature due to radiational cooling at night when the
sun has set.sun has set.
• We start with a clear evening with moderate or high humidity and lightWe start with a clear evening with moderate or high humidity and light
winds. As the sun's energy diminishes at dusk, the ground surfacewinds. As the sun's energy diminishes at dusk, the ground surface
radiates its heat away faster than it can gain it from other sources,radiates its heat away faster than it can gain it from other sources,
and thus its temperature drops. The cool surface then cools the air inand thus its temperature drops. The cool surface then cools the air in
contact with it. Throughout the night, the surface and overlying aircontact with it. Throughout the night, the surface and overlying air
continue to cool (unless warmer air moves in, or a cloud deck formscontinue to cool (unless warmer air moves in, or a cloud deck forms
above); the degree of cooling depends on several factors includingabove); the degree of cooling depends on several factors including
cloud cover, wind speed and number of hours of darkness.cloud cover, wind speed and number of hours of darkness.
• If the air reaches its condensation, orIf the air reaches its condensation, or dewpointdewpoint, temperature during, temperature during
that nocturnal cooling, fog will form. A wet surface — moist soil orthat nocturnal cooling, fog will form. A wet surface — moist soil or
pools of standing water — significantly increases the chances ofpools of standing water — significantly increases the chances of
radiation fog formation, so the radiation fog potential is high after aradiation fog formation, so the radiation fog potential is high after a
rainfall, particularly if followed by a cold front which clears the skiesrainfall, particularly if followed by a cold front which clears the skies
and lowers the air temperature. Radiation fog is not as common overand lowers the air temperature. Radiation fog is not as common over
water surfaces since the cooling of the water surface by radiation iswater surfaces since the cooling of the water surface by radiation is
much slower during the night than the cooling occurring over the land.much slower during the night than the cooling occurring over the land.

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• Advection fogsAdvection fogs are fogs formed when air moves either overare fogs formed when air moves either over
a cooler surface or over a warmer, moist surface, and as aa cooler surface or over a warmer, moist surface, and as a
result the air mass reaches saturation. Most often thisresult the air mass reaches saturation. Most often this
occurs when a moist air mass moves over a cold surface,occurs when a moist air mass moves over a cold surface,
such as a large, cold body of water or snow/ice cover,such as a large, cold body of water or snow/ice cover,
whose temperature is below the dew point of the advectingwhose temperature is below the dew point of the advecting
air mass, and its lowest reaches are cooled to condensation.air mass, and its lowest reaches are cooled to condensation.
• The formation of advection fogs is enhanced when theThe formation of advection fogs is enhanced when the
distance (fetch) over which the advecting air moves is large.distance (fetch) over which the advecting air moves is large.
A low wind speed heightens the likelihood as the air remainsA low wind speed heightens the likelihood as the air remains
in contact with the surface long enough to cool the air layerin contact with the surface long enough to cool the air layer
sufficiently. Advection fogs are often persistent since thesufficiently. Advection fogs are often persistent since the
weather situation that forms them can last a day or more.weather situation that forms them can last a day or more.
Usually, either a frontal passage and change of air-mass orUsually, either a frontal passage and change of air-mass or
a major change in wind direction are needed for dissipationa major change in wind direction are needed for dissipation
of advection fog to take place.of advection fog to take place.

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• UpslopeUpslope andand valley fogsvalley fogs are two special casesare two special cases
• They form when air moving in hilly or mountainous terrain cools toThey form when air moving in hilly or mountainous terrain cools to
condensation.condensation.
• For example, when air tries to move over a terrain obstacle, it cools to someFor example, when air tries to move over a terrain obstacle, it cools to some
degree as it rises, the degree of cooling depending on the amount of rise.degree as it rises, the degree of cooling depending on the amount of rise.
During that cooling, if the air temperature falls below the dew point, theDuring that cooling, if the air temperature falls below the dew point, the
resulting condensation will form a cloud. If that cloud hugs the ground, itresulting condensation will form a cloud. If that cloud hugs the ground, it
becomes fog at that surface. For example, an air mass moving over a waterbecomes fog at that surface. For example, an air mass moving over a water
body may gain moisture until it is very near its saturation level. While stillbody may gain moisture until it is very near its saturation level. While still
over the water, it may not cool enough to reach condensation, but whenover the water, it may not cool enough to reach condensation, but when
forced over the shore, the rise from water level may cause enough coolingforced over the shore, the rise from water level may cause enough cooling
to form a cloud/fog.to form a cloud/fog.
• When we see clouds hugging a mountain summit, you can be sure it isWhen we see clouds hugging a mountain summit, you can be sure it is
foggy on the mountain it embraces. In areas like the raincoast of the Pacificfoggy on the mountain it embraces. In areas like the raincoast of the Pacific
Northwest of North America, upslope movement of moist Pacific air canNorthwest of North America, upslope movement of moist Pacific air can
cause extensive fog at higher elevations which disappears when the aircause extensive fog at higher elevations which disappears when the air
descends on the lee of the ranges.descends on the lee of the ranges.
• Valley fogs form when the air near the terrain heights cools — usually byValley fogs form when the air near the terrain heights cools — usually by
radiation at night — and descends through its greater density into theradiation at night — and descends through its greater density into the
surrounding valleys, flowing like water. Pooled in the valley, the cold air maysurrounding valleys, flowing like water. Pooled in the valley, the cold air may
condense the water vapor present into a fog which fills the valleys to acondense the water vapor present into a fog which fills the valleys to a
depth. Satellite photographs show the dramatic regionality of valley fog,depth. Satellite photographs show the dramatic regionality of valley fog,
bright fingers of fog lying between mountain ridges.bright fingers of fog lying between mountain ridges.

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• frontal fogfrontal fog, which may also be called, which may also be called precipitation fogprecipitation fog..
This fog type generally occurs when rain falling fromThis fog type generally occurs when rain falling from
warm air aloft evaporates at or near the surface underwarm air aloft evaporates at or near the surface under
light wind conditions. The evaporating precipitation as itlight wind conditions. The evaporating precipitation as it
falls through colder air thus increasing the surface air'sfalls through colder air thus increasing the surface air's
moisture burden until condensation is achieved. Suchmoisture burden until condensation is achieved. Such
fogs are most common in the vicinity of warm orfogs are most common in the vicinity of warm or
stationary fronts, but they can form at cold fronts as well.stationary fronts, but they can form at cold fronts as well.
However, cold fronts generally move and mix too quicklyHowever, cold fronts generally move and mix too quickly
to allow the condition to persist for long.to allow the condition to persist for long.
• Precipitation fogPrecipitation fog can also occur under other conditions,can also occur under other conditions,
such as beneath an area where rain falls from air drivensuch as beneath an area where rain falls from air driven
upslope over terrain. It may also form briefly in areasupslope over terrain. It may also form briefly in areas
where hot surfaces are quenched by showers. In thiswhere hot surfaces are quenched by showers. In this
situation the hot surface forces evaporation of the rainsituation the hot surface forces evaporation of the rain
hitting it and the vapor mixes with the cool airhitting it and the vapor mixes with the cool air
surrounding the falling rain to become over-saturated.surrounding the falling rain to become over-saturated.

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• A Foggy SituationA Foggy Situation
• Because of its obscuring nature, fog is oftenBecause of its obscuring nature, fog is often
used as a metaphor for the hidden,used as a metaphor for the hidden,
unknown or unclear. Poet Emilyunknown or unclear. Poet Emily
Dickenson's last words were reported to be:Dickenson's last words were reported to be:
"The fog is rising." Its image fills literature"The fog is rising." Its image fills literature
and poetry and song. But fog as fog has itsand poetry and song. But fog as fog has its
own place in the hearts of many writers. Myown place in the hearts of many writers. My
favorite, and perhaps the best known, isfavorite, and perhaps the best known, is
Carl Sandburg's short poemCarl Sandburg's short poem FogFog..
• Even before my eyelids rose to greet theEven before my eyelids rose to greet the
dawn, I knew fog had settled over the city.dawn, I knew fog had settled over the city.
The repeated moan of the Victoria HarbourThe repeated moan of the Victoria Harbour
foghorn had been my distant town crier,foghorn had been my distant town crier,
spreading the news over the sleepy city.spreading the news over the sleepy city.
When I first looked out my window, the fogWhen I first looked out my window, the fog
was thick around me and closed around thewas thick around me and closed around the
neighborhood. Trees and buildings only tensneighborhood. Trees and buildings only tens
of meters distant were swallowed by theof meters distant were swallowed by the
fog's embrace. I settled back and enjoyed afog's embrace. I settled back and enjoyed a
few more minutes of sleep.few more minutes of sleep.
The fog comes on little cat feet. It sits looking over harborThe fog comes on little cat feet. It sits looking over harbor
and city on silent haunches and then moves on.and city on silent haunches and then moves on.
Carl Sandburg, FogCarl Sandburg, Fog