Freezing Fog, Rime observed in Hulaco Wednesday morning: What is it and how does it form?

Check out these photos of freezing fog (in the distance) and rime (on the plants and fence) sent to WHNT News 19 by Greg Hudson in Hulaco.

Wednesday morning low temperatures fell into the mid 20s, and morning dewpoints were in the mid-20s as well.

As a result, any water vapor present in the atmosphere condensed into super-cooled water droplets, or tiny drops of liquid water that were colder than the freezing point.

Fun fact about water: Liquid water needs a solid surface that resembles an ice crystal — known as a freezing nucleus (multiple surfaces are known as freezing nuclei) — in order for it to change over to ice.

If freezing nuclei are not present, the water droplets will remain in a supercooled yet liquid state — in other words, they will stay liquid even below the freezing mark!

That’s where the term “freezing fog” comes in: When the air is sufficiently cooled to the dewpoint, any water vapor present condenses out into liquid water. But if the air temperature and the dewpoint are both below freezing, this supercooled air can remain liquid until it comes in contact with a surface, like a car or plants — freezing into ice crystals on contact.

Wednesday morning, fog developed in northern Alabama just as temperatures and dewpoints dropped below freezing. This fog remained liquid as it formed, but froze on impact with any surface with which it came in contact. This frosty layer is known as “rime” and it appears as an opaque, milky-white granular layer on exposed objects.

Other types of frost include glaze as well as hoarfrost, as seen in the photos below.

Glaze Frost (Photo: Creative Commons)

Hoarfrost in German (Photo: Dschwen/Creative Commons)

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