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Advection fog can be formed when..
  • A
    moist air is lifted by hills or mountains
  • B
    moist air flows over a surface that is colder than the air
  • C
    moist air is cooled during a calm night
  • D
    moist air flows over water that is warmer than the air

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.

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