Not the Earth from Space but fern frost imaged by Martin McKenna (nightskyhunter). ©Martin McKenna, shown with permission.
At 03.00 UT on the morning of 1st December the temp dropped to – 5°C here in Maghera, Northern Ireland making it the most bitter night of the season to date. A severe frost had already coated everything in white not long after sunset the previous evening. However it was during the early morning hours that I spotted this display of 'fern frost' covering the blue roof of the car in a complex array of leaf patterns which was simply gorgeous when illuminated by the near full Moon high in the sky.

Frost grows on the ground and other surfaces in all manner of forms but all are based on the fundamental hexagonal structure of ice. The particular form taken depend on many thins, the humidity, air temperature, whether the sky is clear and on the texture and conductivity of the surface.

Fern frost, so named for its leafy fern like swathes, is best seen on window panes and other smooth glassy surfaces after a cold night.

Fern frost formation starts when the surface cools below that of the surrounding temperature by radiating its heat out to the empty night sky.

Water molecules from the air condense onto rough (at an atomic level) nuclei – scratches, pits, dust particles. Once a tiny ice crystal has formed it spreads across the cold surface. Crystal growth is fastest at edges rather than on the large smooth planes of facets. We see large single crystal shapes because the initial growth nuclei are far apart. It’s possible that the more polished and cleaner is your car, the larger the fern shapes!

The growth is dendritic, treelike, along the hexagonal crystal directions to give the characteristic ribbed appearance. The complex but overall symmetric shapes of snowflakes are similarly produced.

Why do the crystal intersections have sweeping gentle curves to give the fern leaf shapes? I don’t know. But whatever controls the overall shapes is a general effect, at right are more fern shapes pictured on my own car after a frosty night a few years ago.


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