Crystalline Entities – Snow, Hoarfrost, and Rime Ice

Chefren Night Canada Hoarfrost
Hoarfrost under a snow covered mountain

Waking up after the first heavy snowfall has always been a magical experience for me – a white cloak transforms familiar objects into alien shapes that look like lumbering snow-creatures. Snowfalls present us with macroscapes (a scene we can view with the naked eye like a landscape) and microscapes (scenes that are best seen with a magnifying lens).1 With luck the conditions will be right so that I can see the intricate, feathery ice crystals – the microscapes – that make up the enveloping blanket.2

Snow crystal symmetry hexagon flake
Classic, flat snow crystal showing 60° angle between arms

6 is the magic number

Schoolchildren around the world know that a snow crystal (AKA snowflake3) has 6 “arms,” but that is just the tip of the iceberg. Scientists who study snow crystal formation4 identify at least 12 different types with different shapes ranging from flat and plate-like to traditional star shapes to columns and needles, yet the 6-fold symmetry of 60°s between arms or sides is still recognizable even if arms are not apparent. This symmetry reflects the underlying properties of water molecules when they bind together in the snow crystal, and I am continually in awe that the underlying molecular geometry is preserved in the shape of a crystal a child might try to catch on her tongue during a snowfall.

"No matter how far we come, our parents are always in us" – Brad Meltzer

The different shapes are formed at different humidities and temperatures, and as the crystals swirl around in the atmosphere and grow, they encounter many different conditions. So the mature one that finally lands on your jacket is the product of many different parenting styles, and its shape reflects that varied history. A columnar snow crystal, having begun its life at one temperature and humidity, may develop plates that cap it as it falls and passes through a different temperature. Many of the details of snow crystal formation are still poorly understood, such as why column formation can occur at some freezing temperatures but not at others. Snow crystals can be finicky things.

Ice crystal column plate flake
Side view of a snow crystal column capped with crystal plates.

"We are stardust", said one snow crystal said to another.

For snow crystals to form, moisture must be present, and the temperature must be low enough to condense or gather it together as ice. A third, less familiar component – microscopic dust particles in the atmosphere – is also needed to begin the process. Water vapor molecules collect as ice on the dust, or the particle induces a tiny, cold water droplet to freeze. If you look at the center of a snow crystal, you will find a tiny dust nucleus which may be clay, dirt, ash, or even a microorganism.5

Crystal cousins

Unattached and uninfluenced by other objects aside from their dust nuclei, natural snow crystals form in free-fall through the atmosphere, during which they can form the beautiful symmetry some crystals show. However, many other ice crystals, such as rime ice or hoar frost, form on surfaces under a variety of conditions. The surfaces serve the same role as the dust particles for snow crystals and provide starting points for crystals that can reach impressive sizes, as seen in the first photograph above. These crystals share the same 60° angle between their sides or arms even as they grow quite large.

Hoarfrost lichen hexagon
Hoar frost on bark and lichen. The magnified inset shows the characteristic 60° angle.

Frost, and the larger hoarfrost, form on calm, freezing nights when water vapor contacts the cold ground, grass, or other surfaces and assumes the familiar icy plates, needles, and columns up to several inches long. The water vapor can come from a nearby body of water or even from an animal’s den, forming hoarfrost around the opening, so good places to look for hoarfrost are near a stream or other water source or by the entrance to an animal’s home.

Hoarfrost opening animal den
A rim of hoarfrost around the opening to an animal's den

Blowing in the wind

Hoarfrost crystals don’t usually have a dominant orientation since they form with little or no air movement; they are often haphazardly deposited on cold surfaces. Rime ice, on the other hand, forms under the influence of cold wind and points into the wind, despite looking like wind-blown hair which points away from the wind.

Dog hair fur wind
Herschel's hair blowing away from the wind
Rime ice wind
Rime ice forming into the wind

Supercooled water droplets,6 below 0°C (32°F), carried by wind strike a cold object, like the branches above, and instantly freeze into rime ice. Because most of the droplets freeze on contact, the rime accumulates on the upwind side, leaving little moisture in the wind to freeze on the downwind side. Under certain conditions, for example, in icy fog on Mount Washington in New Hampshire, rime ice can become more than a foot thick.

It's inevitable your environment will influence what you do. – Duncan Sheik

Ice crystals glass window
Ice crystal "script" on a glass window

Ice crystals can be induced to disobey and not follow the 6-sided rule, particularly on smooth surfaces. You may have noticed curving, ice feathers on glass that resemble beautiful but indecipherable handwriting. Perhaps the crystals crisscross in multiple, straight lines like contrails in the sky. These variations from the norm are due to “imperfections” on the glass surface that cause crystalization to proceed seemingly willfully. On close inspection, one can sometimes find the culprit – in the photograph of crisscrossing lines below, spiders had left behind silken draglines on the glass that produced the straight, crystalline lines.

Ice crystals glass window
Ice crystals on a glass window follow the straight, silken draglines of spiders

There’s no business like snow business

Tunneling through many fewer than a quadrillion snowflakes

Scientists have estimated that a quadrillion (that’s 1 followed by 15 zeros or 1,000,000,000,000,000)7 snowflakes fall each second around the world. Multiply that by the millions of years snow has been falling, and consider that number the next time you hear, “No two snowflakes are alike.” Then go out into the cold with a magnifier in hand and look for some of the quadrillions of microscapes waiting to be discovered.

Notes

1 Macroscape and microscape don’t appear in the dictionary, but they should.

2 I and all other living things on earth are lucky to live on a planet with liquid water. A doomed world, like in Kurt Vonnegut’s Cat’s Cradle, frozen by the military-invented ice-nine that is solid at room temperature, would be uninhabitable. Our planet circling the sun is in a “Goldilocks Zone” where liquid water exists enabling life.

3 Scientists refer to “snow crystals” as the individual crystals ranging from 6-armed, beauties whose paper replicas are hung on Christmas trees to tiny ice needles or columns. A “snowflake” can be a single snow crystal or a giant, fluffy clump of many snow crystals that combine in the sky before falling to Earth.

4 Scientists have been fascinated by snow crystals for centuries, including the Vermont farmer Wilson A. Bentley who took thousands of remarkable snow crystal photographs starting in the 1800s. There remains a considerable mystery about the details of how they form.

5 There may be multiple particles scattered throught the snow crystal in addition to the central nucleus. These other particles further influence the shape of the crystal.

6 Recall that snow crystals form around a dusty nucleation particle. In the absence of these nuclei, the tiny, individual, water droplets in a freezing fog may not become ice until they contact a cold surface, despite being below the freezing temperature.

7 A quadrillion is almost an unimaginably large number. Consider that the population of one hundred thousand Earths combined would be close to a quadrillion people.

References

Thanks

Special thanks to Zoe Weil for her editorial expertise and to Forest Barkdoll-Weil for the use of his photograph.

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Rae
Rae
1 year ago

60 degrees!!?!! LOVE this one…the quotes were great….the photos magical!!! And the icing on the frosty cake was the tunneling video! That was fabulous…Big smile over here. I have been a fan of hoarfrost since my very first sighting in Minnesota….covering an apple orchard. Thanks for more blog joy!

Zoe Weil
Zoe Weil
1 year ago

Thank goodness you made it through that tunnel. I was laughing so hard as you emerged that I forgot to be scared you’d suffocate!

Carol
Carol
1 year ago

Thank you for these beautiful photographs of macro and micro snow!
And for mentioning one of my favorite books, Cat’s Cradle.

Margie Patlak
1 year ago

Amazing information and photographs, thanks for sharing! But I’m mulling over your footnote number six because it’s my understanding that although it contains finer droplets than rain or snow, the droplets in fog also form by water latching onto dust particles although the dust particles may also be finer and perhaps minerals. Do you have more information you can share about this?

Margie Patlak
1 year ago

Thanks for that clarification Edwin! So if I understand things correctly, the sea smoke so prevalent in coastal Maine in winter is due to nucleation, but the thin layer of ice seen on the rocks in the bay could be due to ice fog. Usually I get an email notification when I respond to blogposts and the blogger responds to mine. But that didn’t happen with your response, unless it ended up in my spam folder. Is that something you can set up? I look forward to more of your blogposts!

Margie Patlak
1 year ago
Reply to  Edwin Barkdoll

And more great answers, thanks!

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