Brett's Weather Blog: The causes of ice frost, hoarfrost and freezing fog

Hello everyone,

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The above picture was a common site Thursday morning. We had a rare occurrence of freezing fog. Freezing Fog simply occurs when there is fog and the temperatures are below 32 degrees. But you might have noticed the fog coated everything in white. This is why its sometimes called a white frost. Other common names are Hoarfrost or Ice Frost.

So what makes this icy fog different?

Hoarfrost is not the same thing as the frost that we scrape off our car windows & windshields. Hoarfrost often looks feathery or looks like needles. The below picture is a great example of that.

First of all we have to understand how frost forms. Frost forms easily on grass and windows when the temperature reaches the frost point temperature. Thursday morning, the conditions were ideal for this type of frost to form; calm wind, clear sky, a long cold night.

It also helps that much of Green Country is covered in vegetation, a perfect surface for what is called radiational cooling, or the earth's release of stored heat. So when ice particles are forming, they take a shape that is a function of the difference between the air temperature and the surface temperature where the ice particles are forming.

Maybe this definition from Glossary of Meteorology will help:

Hoarfrost -- A deposit of interlocking ice crystals formed by direct sublimation on objects, usually those of smaller diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wire, poles, etc.

We saw a lot of hoarfrost that took on the shapes of needles or crystals. Here is another definition from the Glossary of Meteorology that helps explain why frost forms in that needle or crystal shape.

Crystalline frost -- Hoarfrost that exhibits a relatively simple macroscopic crystalline structure; to be distinguished from amorphous frost. Nakaya classifies the crystalline frosts into five forms: (a) needle, (b) feather-like, (c) plate, (d) cup and (e) dendritic. Such ice forms are typically developed as a result of sublimation at temperatures well below 0C; the degree of supersaturation and temperature controlling the form.

Sublimation is when something goes from a solid to a gas without passing through the liquid state. In the case of hoarfrost or white frost, this is really de-sublimation since its going from water vapor to the solid ice without becoming a water droplet.

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