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Fog Formation Anthony R. Lupo Lab 8 Atms 4310. Fog Formation  Definition: Fog  a cloud (usually stratiform) close to, or at, the ground that impairs.

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Presentation on theme: "Fog Formation Anthony R. Lupo Lab 8 Atms 4310. Fog Formation  Definition: Fog  a cloud (usually stratiform) close to, or at, the ground that impairs."— Presentation transcript:

1 Fog Formation Anthony R. Lupo Lab 8 Atms 4310

2 Fog Formation  Definition: 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).

3 Fog Formation  Formation Processes  Radiation Fog (Fair weather fog)  - due to nighttime IR cooling, the air cools to dewpoint  Surface radiational cooling  ground cools and shallow nighttime PBL forms (radiation or ground fog).

4 Fog Formation  Sounding (Thanks LCH WFO)

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

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

7 Fog 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

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

9 Fog 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).

10 Fog Formation  Winter situation (upper plains / Northeast)

11 Fog Formation  foul 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)

12 Fog Formation  Fall situation

13 Fog Formation  Transport of moist air over sloping terrain (adiabatic process) “upslope fog”  Can be a combination of factors  e.g. advection and radiation  advection or precipitation.

14 Fog Formation  Fog dissapative processes:  lateral or vertical mixing w/drier air  can occur with the onset of strong wind (radiation fog)  can occur with the change in air mass

15 Fog Formation  4) solar heating (fog “burns off”)  5) heavy rain can strip suspended droplets.

16 Fog 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.

17 Fog Formation  Haze:  Restrictive to visibility at times:  Generally forms at conditions less than saturation.

18 Fog Formation  May be composed of solid particles (Mexican fires of Mid-May, 1998)  Typically in association with a quasi- stationary stagnant High pressure area.

19 Fog Formation  The End!

20 Fog Formation  Questions!  Comments!  Criticisms! 


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