Hoar Frost

Fern Frost on hood of my van (2/14/14).

I recently posted an image of fern frost which I discovered on the hood of my van one frosty morning last week. I often see this type of frost on cold mornings on the interior of the storm windows of my house (if you live in a newer house with energy efficient double pane windows you’re less likely to see frost on your windows).

But there are other types of frost, and they form in different ways. The two that I most frequently encounter are fern frost and hoar frost (hoar rhymes with the boar in wild boar).

Fern frost forms when moist air condenses on glass or other similarly smooth objects (like the paint job of my van) forming dew which in turn freezes as the temperature drops below freezing. I’ve most often seen it in more angular formations rather than the curvy, flowing pattern at right.

Typically, the frost follows tiny cracks or scratches in the glass’s surface. The curvy aspect of the frost in the image may be due to tiny scratches or residue left by my, admitedly infrequent, washings of the van. The circular motion of washing the surface of the paint may have left behind grooves or residual soap on the paint (probably circular patterns of dirt), the frost following the pattern on the paint’s surface.

Another likely reason for the curved pattern may be due to patterns formed by air currents while driving down the road at highway speeds, the air forming tiny vortices which create patterns on the paint’s surface. Either theory sounds plausible.

I usually encounter hoar frost well into winter. Hoar frost is formed when a particular surface (a tree trunk, wooden railing) is already very cold causing the water vapor in warmer moist air to freeze upon contact with the much colder surface. It most often has a spiky appearance.

Hoar frost on the railing of the boardwalk here at the museum.
A closer look at hoar frost.

The hoar frost in the above image is reminiscent of cedar leaves.

Notice the similar structure of these red cedar leaves.

Although I’ve personally had enough of winter, we will probably have a chance to see at least a few more displays of either hoar or fern frost this season (certainly not this week, 60s and 70s!). Look for it in the morning after especially humid nights with low temperatures and little or no wind. Get down and take a close look at the structure of the frost. Take some pictures. Let me know what you find out.

Have fun!

1 response to Hoar Frost

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    Judy Overby says:

    Very interesting and beautiful; and I always thought Jack was responsible:)

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