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Fog checklist
Definition:
 Visible aggregate of minute droplets, or crystals suspended near the sfc
 Visibility < 5/8 sm (1 km); if > 5/8 sm, called mist (BR)
Formation:
 Cooling air to its dew point (by radiation, advection, or lifting); T 
 Adding water vapor (by evaporation, mixing, or human activity); Td 
Dissipation:
 sunshine
 brisk winds
 heating of air
 changes of formation conditions
Types:
 radiation fog, advection fog, upslope fog, frontal fog, steam fog, ice fog, snow fog
1. Radiation Fog

Formation:

nighttime radiation cooling of air from ground upward

occurs late night; deepest around sunrise

generally short duration (< 24 hrs)

Patchy and localized

Favorable conditions

clear skies under Highs

small T-Td at sunset

light winds (<10 kts) or slight turbulence (if no wind, then promote dew)

mixing ratio value increases with height  dense persistent fog

Wet surface: less cooling is required to form the fog.

local topography: valley or low spot for cold air drainage  valley fog

Condensation nuclei

Stable air or inversion near surface

Dissipation processes

Solar heating: fog “burn off” turbulent mixing and mixing with drier air; dissipate
3-4 hrs after sunrise

Strong wind (> 10 kts)  turbulent mixing
“inward mixing”  fog burn off
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2. Advection Fog

Formation:

warm moist air advected over a cold sfc (land, snow, ice, cold sea and lake)

may persist several days until air mass or wind direction changes

may occur anytime of day and any seasons

advected over large area and may be stopped by land barriers

Favorable conditions:

High RH

Stable stratification near surface

Moderate winds (8-17 kts)

Tair- Tsfc = large

large horizontal temperature gradient

Moist Flux in the lowest levels dTd/dz < 0

Cold surface can be:

Dissipation processes

surface heat

Wind direction changes or air mass changes

stopped by land barriers: wind barrier effect
3. Upslope Fog

Fromation: adiabatic cooling as air rises along a slope

Favorable conditions

Hilly terrain or slopping ground

Upslope or onshore wind

High RH

Increase of moisture with height

Stable air mass

Dissipation Process

air mass or wind direction changes

Dissipates on leeward side due to subsidence

Mix with drier air
4. Frontal Fog:

Prefrontal fog (warm front)


Evaporation of relatively warm rain falling through cold layer
Temperatures ahead of front lower than Td behind the front
2
Cold air nearly saturated

Rain warmer than surrounding air

Stratus fractus often a precursor

Often observed with drizzle
Postfrontal fog (cold front)

Precipitation falling into cold stable air

Light winds

Wet surface


5. Steam fog/Arctic sea smoke

Formation:

Result of evaporation, cold air flowing over much warmer water

Unstable layer near the water surface, will favor evaporation

Instability leads to flurries rather than fog.

Favorable conditions

Early morning or autumn (favorable temperature of water vs air)

In winter in the Arctic over polynias

High RH

Light winds

High concentration of condensation nuclei
6. Ice Fog (sublimation)

Formation:

the air (with an initial temperature < 0C) cool well below its frost point (super
saturation)

Combustion adds moisture and condensation nuclei; near open water and
settlements

Favorable conditions

an adequate moisture supply in low levels: for example near settlements, or power
plants.

air temperature colder than -32C: could be present at lower temperature
depending on the source of crystallisation nuclei available

a low level temperature inversion: to inhibit dispersion of particles

sfc winds < 5 knots : stronger winds will dissipate the water vapor concentration
available for the formation of the ice fog.
7. Snow Fog

formed by advection or evaporation

temperature of the air between -10C and 5C
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