2Fog FormationDefinition: Fog a cloud (usually stratiform) close to, or at, the ground that impairs surface visibility or obscures the sky.Visibility Criterion: (what must be reported METAR)WMO criterion (US reporting generally stricter)< 3 mi (Fog)3 to 5 miles (Fog or haze)5 to 7 (Haze)> 7 mi (no classification necessary).
3Fog Formation Formation Processes Radiation Fog (Fair weather fog) - due to nighttime IR cooling, the air cools to dewpointSurface radiational cooling ground cools and shallow nighttime PBL forms (radiation or ground fog).
5Fog FormationForms underneath cool high pressure (east and southeast side [low-level CAA may be associated with it]).
6Fog FormationCooling from a moist layer near the ground lying beneath a dry inversion (inversion fog) (This also occurs underneath High pressure)
7Fog Formation Precipitation Fog (Foul weather fog) saturation of cool, low-level air by evaporation of relatively warm precipitation (rain fog) this mechanism works better with drizzle – smaller droplets
8Fog Formationwhen associated with fronts this is referred to as frontal fog. More typically associated with warm fronts (ahead of the front.
9Fog Formation Advection Fog Can be caused by the transport of warm moist air over a cool surface (advection fog), a WAA situation (Fair or foul weather fog).
11Fog Formationfoul weather this occurs typically in the warm sector, associated with warm air passing over snow. also can be associated with cool dry air over a warm water body or moist land. (steam fog or sea smoke), and these fogs especially the latter can be quite turbulent or unstable. (Cold highs)
13Fog FormationTransport of moist air over sloping terrain (adiabatic process) “upslope fog”Can be a combination of factorse.g. advection and radiationadvection or precipitation.
14Fog Formation Fog dissapative processes: lateral or vertical mixing w/drier aircan occur with the onset of strong wind (radiation fog)can occur with the change in air mass
15Fog Formation 4) solar heating (fog “burns off”) 5) heavy rain can strip suspended droplets.
16Fog Formation Relative humidity considerations: 1) in the presence of sufficient condensation nuclei fog begins forming at 90 – 95% RH.2) if the air is too clear of condensation nuclei, then droplets are too small for fog.
17Fog Formation Haze: Restrictive to visibility at times: Generally forms at conditions less than saturation.
18Fog Formation May be composed of solid particles (Mexican fires of Mid-May, 1998) Typically in association with a quasi-stationary stagnant High pressure area.