The fog on Thursday morning prompted the National Weather Service in Pittsburgh to issue a freezing fog advisory for some counties where the temperature was at or below freezing.

This was the first time this office has issued a freezing fog advisory, but other National Weather Service offices across the country have been doing this for years and continue to issue this when warranted.

(Photo: IEM)

Freezing fog itself is not a regular occurrence, but it is also not a rare phenomenon and has occurred many times before in western Pennsylvania and other parts of the world.

Fog is essentially a cloud based near ground level. Most clouds are made up of supercooled liquid water droplets that can exist in temperatures as cold as -10 degrees to -40 degrees (aka well below freezing). When these d

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