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Advection fog is most likely to form when:
  • A
    maritime warm air flows over a relatively warmer surface and the wind speed is greater than 15 kt.
  • B
    a mild moist airstream flows over snow covered ground and the wind speed is less than 10 kt.
  • C
    maritime cold air flows over a warmer surface and the wind speed is greater than 15 kt.
  • D
    cold air is forced over higher ground and further adiabatic cooling occurs.

Refer to figure.

ADVECTION FOG. Is formed by the advection of warm, moist air over a cold surface – the air mass is cooled from below giving rise to an inversion. The surface can be land or sea and it can appear by day or night.
CONDITIONS NECESSARY FOR ADVECTION FOG TO FORM

  • WIND SPEEDS. Up to 15 kts to move the air (may be stronger over sea – up to 20 kts).
Wind speeds over 5 kts are sufficient for advection fog formation, but the speeds of around 15 kts provide the conditions for maximum vertical development of advection fog.
  • COLD SURFACE. Colder than the dew point of the air moving over it – to ensure condensation.
  • HUMID AIR. High relative humidity – so that relatively little cooling is required to produce saturation and subsequent condensation.
  • TEMPERATURE DIFFERENCE. The greater the temperature difference between the warmer air and the colder ground => the greater the likelihood of fog formation. (mild air over snow covered surface)

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