Crystalline Entities – Snow, Hoarfrost, and Rime Ice
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
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.
"We are stardust", said one snow crystal said to another.
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.
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.
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.
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 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.
There’s no business like snow business
Tunneling through many fewer than a quadrillion snowflakes
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.
Special thanks to Zoe Weil for her editorial expertise and to Forest Barkdoll-Weil for the use of his photograph.