What is freezing fog?

Weather Blog

Dear Tom,
My weather app several days ago used the term “freezing fog.” What is freezing fog?

—Shirley Foster, Chicago

Dear Shirley,
Freezing fog is regular fog but at a temperature that is below the freezing point of water, 32 degrees. Fog is composed of tiny water droplets. The droplets remain in liquid form even when the temperature drops below freezing because water molecules (in fog droplets) require a surface upon which to coalesce in order to freeze. Lacking this surface, the droplets remain liquid down to about 14 degrees, but below that temperature they can indeed freeze and they are then called ice fog. If the temperature drops to -40 degrees, or lower, all liquid water droplets in fog will freeze and become ice fog. In the case of freezing fog, the fog droplets are supercooled.

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