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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.

  • 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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