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Fog Formation

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 surface visibility or obscures the sky. Visibility Criterion: (what must be reported METAR)

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Fog Formation

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  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! • LupoA@missouri.edu

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