What is Fog?

Fog is defined as cloud that forms at ground level. It is generally accepted that the visibility must also be below 1000m to be classified as fog. (Brandon, 2009[1]) If the visibility is higher than this, it is classified as mist.
Since fog is essentially cloud, it makes sense that the same conditions are needed for fog to form. These include:

  • A presence of water vapour
  • A cooling mechanism
  • Hygroscopic particles (minute particles in the air, such as smoke or dust)

A light wind is also required, otherwise the moisture will decend and form dew on the ground. Wind speeds would normally be between 1 knot and 5 knots. The atmosphere also needs to be relativley stable.


(Wagtendonk et al., 2003[2])

Type Typical Location Time of Day Formation
Radiation Fog Over land as it begins to cool Any time after sunset, to before the sun rises. When the sun is not present, the ground begins to cool. As the ground cools, this will also cause the air to cool, untill the saturation point1 is reached, and moisture condenses.
Advection Fog A cold surface Similar to Radiation fog This type of fog forms when warm moist air moves over a cool surfce. The air then cools, and water condenses forming fog.
Valley Fog In a Valley with a source of water, for example a river. Late in the afternoon/ Evening As warm moist air flows out of a valley, the sides of the valley may have been in shadow for some time, and can be alot cooler. Thus the air is then cooled after passing over these sides, and fog may form if enough moisture is present.
Steaming Fog Over a water source, (e.g. an ocean) Any time If cool dry air moves over a moist surface, it can gain moisture very quickly. If the air reaches its saturation point, fog can form. This can happen with cool dry air moving over a warm, moist sea surface, or in geothermal areas for example.
Frontal Fog Ahead of a warm front Any time This type of fog can form ahead of weak warm fronts, as the wind needs to be light for fog to form. Precipitation falling from the advancing warm air can saturate the air below, causing fog to form.


The Golden Gate bridge in California, USA is a notorious spot for fog formation, which can last well into the day. Watch a time lapse video on a particularly foggy day:

Video embedded from YouTube 05th Oct 2009

In this video you can watch the development of fog over Vancouver, USA during the night:

Video embedded from YouTube 05th Oct 2009
1. Brandon, J. (2009) Cloud, Fog and Precipitation. Retrieved 05 Oct 2009 from the World Wide Web:
2. Wagtendonk, W. J., Boys, S. & Wagtendonk, J. (2003) Meteorology for Professional Pilots.Bay of Plenty, NZ: Aviation Theory Centre.

Want to know more?

Wikipedia - Fog
More general information on fog.

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