Fog

[Ref Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 12-15]

Fog is a cloud that is on the surface. However, formation of fog could be diabatic/adiabatic process depending on the types (and cloud’s formation is adiabatic process).

It typically occurs when the temperature of air near the ground is cooled to the air’s dew point. At this point, water vapor in the air condenses and becomes visible in the form of fog.

Important

Formation of fog requires some turbulence

If there were no turbulence, with the dewpoint and temperature the same, we would only get dew on the grass (hence the name dewpoint), and not the fog.

 

Common types of fog

Fog is classified according to the manner in which it forms and is dependent upon the current temperature and the amount of water vapor in the air.

  • Radiation fog
    • temperature drops to dew point due to terrestrial radiation (formation similar to dew – by longwave radiation loss – e.g 紅外輻射), but
    • clear clear, cool nights, with relatively little to no wind present (max. 5km/hr)
      usually forms at night AND between late autumn and spring
    • Diabatic process
    • notes: should the radiation fog is less than 20 feet thick, we call that as the “ground fog
    • notes: should the temperature below -32C (-25F), water content in fog cloud be in ice form – “ice fog
    • notes: radiation fog is closely related to temperature inversion
  • Advection fog
    • temperature drops to dew point
    • require wind of up to 15 KT
      (above 15KT may cause low stratus clouds instead)
    • (warm, moist) air passes over cool surface – the term “advection” refers to horizontal movement
    • Diabatic process
  • Upslope fog
  • Precipitation fog
  • Steam fog
    • (cold,dry) air moves over warm water
      (what we see right in front of us when we exhale on a cold day!)
    • a.k.a Sea smoke
    • Be notice that low-level turbulence and icing are commonly associated with steam fog
    • Adiabatic process
      Notes:
      BOTH Fog and Mist affect ground level. However,
      Mist – the horizontal visibility remains above 1000 metres. (>1km)

 

Forms of Condensation

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