Fog FormationFog Formation

Anthony R. LupoAnthony R. Lupo

Lab 8Lab 8

Atms 4310Atms 4310

Fog FormationFog Formation

Definition: Fog Definition: Fog a cloud (usually stratiform) close to, or at, a cloud (usually stratiform) close to, or at, the ground that impairs surface visibility or obscures the the ground that impairs surface visibility or obscures the sky.sky.

Visibility Criterion: (what must be reported METAR)Visibility Criterion: (what must be reported METAR)

WMO criterion (US reporting generally stricter)WMO criterion (US reporting generally stricter)

< 3 mi (Fog)< 3 mi (Fog) 3 to 5 miles (Fog or haze)3 to 5 miles (Fog or haze) 5 to 7 (Haze)5 to 7 (Haze) > 7 mi (no classification necessary).> 7 mi (no classification necessary).

Fog FormationFog Formation

Formation ProcessesFormation Processes

Radiation Fog (Fair weather fog)Radiation Fog (Fair weather fog)

- due to nighttime IR cooling, the air cools to - due to nighttime IR cooling, the air cools to dewpointdewpoint

Surface radiational cooling Surface radiational cooling ground cools and ground cools and shallow nighttime PBL forms (radiation or ground shallow nighttime PBL forms (radiation or ground fog). fog).

Fog FormationFog Formation

Sounding (Thanks LCH WFO)Sounding (Thanks LCH WFO)

Fog FormationFog Formation

Forms underneath cool high pressure (east Forms underneath cool high pressure (east and southeast side [low-level CAA may be and southeast side [low-level CAA may be associated with it]).associated with it]).

Fog FormationFog Formation

Cooling from a moist layer near the ground Cooling from a moist layer near the ground lying beneath a dry inversion (inversion fog) lying beneath a dry inversion (inversion fog) (This also occurs underneath High (This also occurs underneath High pressure)pressure)

Fog FormationFog Formation

Precipitation Fog (Foul weather fog)Precipitation Fog (Foul weather fog)

saturation of cool, low-level air by saturation of cool, low-level air by evaporation of relatively warm precipitation (rain evaporation of relatively warm precipitation (rain fog) this mechanism works better with drizzle – fog) this mechanism works better with drizzle – smaller dropletssmaller droplets

Fog FormationFog Formation

when associated with fronts this is referred to as when associated with fronts this is referred to as frontal fog. More typically associated with warm frontal fog. More typically associated with warm fronts (ahead of the front.fronts (ahead of the front.

Fog FormationFog Formation

Advection FogAdvection Fog

Can be caused by the transport of warm Can be caused by the transport of warm moist air over a cool surface (advection moist air over a cool surface (advection fog), a WAA situation (Fair or foul weather fog), a WAA situation (Fair or foul weather fog).fog).

Fog FormationFog Formation

Winter situation (upper plains / Northeast)Winter situation (upper plains / Northeast)

Fog FormationFog Formation

foul weather foul weather this occurs typically in the this occurs typically in the warm sector, associated with warm air warm sector, associated with warm air passing over snow.passing over snow.

also can be associated with cool dry air also can be associated with cool dry air over a warm water body or moist land. over a warm water body or moist land. (steam fog or sea smoke), and these fogs (steam fog or sea smoke), and these fogs especially the latter can be quite turbulent or especially the latter can be quite turbulent or unstable. (Cold highs)unstable. (Cold highs)

Fog FormationFog Formation

Fall situationFall situation

Fog FormationFog Formation

Transport of moist air over sloping terrain Transport of moist air over sloping terrain (adiabatic process) “upslope fog”(adiabatic process) “upslope fog”

Can be a combination of factorsCan be a combination of factors

e.g. advection and radiatione.g. advection and radiation advection or precipitation.advection or precipitation.

Fog FormationFog Formation

Fog dissapative processes:Fog dissapative processes:

lateral or vertical mixing w/drier airlateral or vertical mixing w/drier air

can occur with the onset of strong wind can occur with the onset of strong wind (radiation fog)(radiation fog)

can occur with the change in air masscan occur with the change in air mass

Fog FormationFog Formation

4) solar heating (fog “burns off”)4) solar heating (fog “burns off”)

5) heavy rain can strip suspended 5) heavy rain can strip suspended droplets.droplets.

Fog FormationFog Formation

Relative humidity considerations:Relative humidity considerations:

1) in the presence of sufficient 1) in the presence of sufficient condensation nuclei fog begins forming at condensation nuclei fog begins forming at 90 – 95% RH.90 – 95% RH.

2) if the air is too clear of condensation 2) if the air is too clear of condensation nuclei, then droplets are too small for fog.nuclei, then droplets are too small for fog.

Fog FormationFog Formation

Haze:Haze:

Restrictive to visibility at times:Restrictive to visibility at times:

Generally forms at conditions less than Generally forms at conditions less than saturation.saturation.

Fog FormationFog Formation

May be composed of solid particles May be composed of solid particles (Mexican fires of Mid-May, 1998)(Mexican fires of Mid-May, 1998)

Typically in association with a quasi-Typically in association with a quasi-stationary stagnant High pressure area.stationary stagnant High pressure area.

Fog FormationFog Formation

The End!The End!

Fog FormationFog Formation

Questions!Questions!

Comments!Comments!

Criticisms!Criticisms!

LupoA@missouri.eduLupoA@missouri.edu