Frosty Car

Last updated on September 16, 2023

Frosty Car: Exploring the Fascinating Phenomenon of Fern Frost

When the temperature drops and the night sky is clear, a captivating phenomenon known as fern frost can be observed on various surfaces, including car windscreens and external bodywork. This intricate frost formation, named for its resemblance to leafy ferns, occurs when the surface cools below the surrounding air temperature, causing water molecules from the air to condense and freeze upon contact.

The process begins with the surface radiating its heat out to the empty night sky, causing it to cool rapidly. As the temperature drops, water molecules in the air seek out nuclei on which they can condense. These nuclei can be scratches, pits, or dust particles present on the surface. Once a tiny ice crystal forms, it spreads across the cold surface, creating a delicate and intricate pattern.

The growth of fern frost is dendritic in nature, resembling tree-like structures that extend along the hexagonal crystal directions. This gives fern frost its characteristic ribbed and branched appearance. The complex shapes of snowflakes are formed through a similar process, as they also grow along these crystal directions.

It is worth noting that fern frost is most pronounced on surfaces that are relatively clean. A nearly spotless car will showcase the patterns with greater clarity, allowing for a more visually striking display. So, if you want to witness this captivating phenomenon in all its glory, it's best to ensure your vehicle is as clean as possible.

The formation of fern frost is highly dependent on temperature and humidity conditions. It typically occurs after a cold night, when the air is moist and the temperature drops below freezing. These conditions provide the ideal environment for water vapor to condense and freeze upon contact with the cold surface.

Interestingly, the growth of fern frost is not uniform across the surface. Crystal growth is faster at the edges of the frost rather than on the large smooth planes. This phenomenon results in large single crystal shapes when the initial growth nuclei are further apart. The combination of intricate dendritic patterns and larger crystal formations contributes to the enchanting beauty of fern frost.

While fern frost was historically observed on old single-glazed window panes, these are now less common. However, car windscreens and external bodywork have become excellent substitutes for studying this captivating natural phenomenon. The smooth surfaces of car windows provide a perfect canvas for the delicate fern-like patterns to form, allowing us to marvel at their intricate beauty.

In conclusion, fern frost is a captivating atmospheric optics phenomenon that occurs when the temperature drops below freezing and water vapor condenses and freezes on a cold surface. The intricate dendritic patterns and larger crystal formations contribute to its mesmerizing appearance. So, the next time you wake up to a frosty morning, take a moment to appreciate the delicate beauty of fern frost on your car's windows or bodywork. It's nature's way of turning a cold winter's day into a work of art.

Frosty Car Roof Fern frost on a car roof imaged by Richard Osborn (site) in Nottinghamshire, England. �Richard Osborne, shown with permission

Fern frost, so named for its leafy fern like swathes, is best seen after a cold night on old single-glazed window panes. They are now hard to come by but car windscreens and external bodywork make fine substitutes.

The frost formation starts when the surface cools below that of the surrounding air 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 when the initial growth nuclei are far apart. (Almost) clean cars make the best patterns

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

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Reference Atmospheric Optics

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