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Name: Brian
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Question:
At 30 degrees below zero, why do we have fog in Fairbanks, Alaska?


Replies:
Brian,

Fog (or clouds) occur when the temperature reaches the dew point (or frost point, which may be the case in your situation). For air with very little water vapor content (low absolute humidity), much lower temperatures are required to create visible fog. That does not mean that no water droplets or ice crystals have been forming before that, but you need a certain concentration of them before visibility is reduced to the point that it is detected as fog.

The aerosols that water or ice form on in the atmosphere change the properties of the solution (raise the surface tension) such that it is more difficult for water vapor in the air to condense on the water/aerosol solution. This results in a lower temperature than dew or frost point (which considers only pure water effects) being needed for visible water droplets or ice crystals to occur.

Ice fog requires particularly low temperatures if the air is very "dry" (low water vapor content), as it obviously requires lower temperatures to produce ice crystals than it normally does to produce water droplets.

It sounds as though the water content in the air at Fairbanks was extremely low, thereby requiring very low temperatures for a visible fog (possibly of the ice type) to form.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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