WEATHER GEEKS LESSON: What is Radiation Fog?

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By Chris Martz | October 14, 2019

One of my favorite things to do is view live webcams of mountain slopes, valleys, beaches, and cities in hopes of catching a cool sunrise, sunset, cool clouds, or atmospheric optical effects.

One of my favorite live views is looking towards a valley from Borestone Mountain in Maine. Yesterday morning, as I watched the timelapse and caught a view of something really cool; radiation fog.

Borestone Mountain.

Radiation fog is a fairly common occurrence in the United States and it only occurs during the overnight hours and it’s statistically most likely to occur during the autumn and winter months.

Certain conditions are necessary for this kind of fog to occur:

  1. There has to be cool air near the ground, and warmer air aloft.
  2. In relation to #1, clear skies with little cloud cover is necessary for the surface to emit longwave radiation back into outer space, thereby allowing air temperatures closer to the surface to cool.
  3. There has to be drier air aloft and moist air near the surface within the “planetary boundary layer.”
  4. If temperatures are able to cool enough, it’ll eventually reach the dew point temperature whereby water vapor must then condensate out into liquid water droplets. This means that the air is saturated.
  5. Light winds are necessary in order to allow the temperature to drop to the dew point, otherwise there is mixing of air.

If all of these conditions are in place, then fog will form. If the air aloft is more moisture-filled and cooled, then the fog may expand and grow vertically.

The most ideal location for radiation fog to form are in valleys which are generally more sheltered from winds and can cool further under clear skies as colder air sinks from the mountains and into the valleys. This is sometimes referred to as valley fog.

Valley and radiation fog formation.

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Categories: Weather

1 reply

  1. Nicely done. Thank you Chris

    Liked by 1 person

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