The poor columns get left out of nearly all snow-crystal discussions, but they are an interesting type. So, to help them out a bit, here's my first column appreciation post.
Let's start with perhaps the most extreme column of all, the Shimizu prism*:
I say 'extreme' because they are so long and thin--sometimes over 1-mm long yet just 0.01-0.02 mm in diameter. These types have so far been found to fall only on the Antarctic Plateau. But in theory, they should be able to form elsewhere. It is like a "whisker" crystal, which Teisaku Kobayashi grew below -50 C on a surface in the lab. The image above shows many other crystals as well, including another solid column crossing the Shimizu prism.
Next, the bullet rosette:
The bullet rosette is most often found below -25 C in high cirrus clouds. It is an example of a polycrystal; in this case, a frozen droplet that froze into several distinct crystals (one for each "bullet").
Next, one of my favorites, the scroll column (though the picture doesn't quite do it justice):
In this form, the sides of the crystal seem to fold inward, like a scroll.
Finally (for now, anyway), the ubiquitous hollow column:
The funny banding you see (horizontal lines inside the 'hollow') is a mystery.
There are many other columnar forms, many of which are in the following diagram (as with all images here, click on it to see it enlarged)**:
One neat thing about the columnar forms is that you can see roughly exact replicas of them in hoarfrost. The Shimizu prism may be hard to find, but the others are common if you look closely.
* Images are from the Magono & Lee collection, used in their paper: Meteorological classification of natural snow crystals. J. Fac. Sci., Hokkaido Univ., Ser. VII 4, 321–335.
**Drawings are based on those in Kikuchi, Kameda, Higuchi, and Yamashita: A global classification of snow crystals, ice crystals, and solid precipitation based on observations from middle latitudes to polar regions. Atmos. Res. 132-133 (2013) 460-472.
Black cars remain my favorite place to observe frost patterns. Here are a few I saw on one car today.
The straight lines of frost are more common when it is drier. And it has been quite dry here due to the very low temperatures.
These pictures show an interesting mixture of straight lines and curved boundaries.
And finally, the windshield had a pattern that resembled a hilly landscape.
But the pattern is actually quite flat. The frost is playing mind games on us, presenting an optical illusion of 3D topography.
As with all cases of frost on surfaces, the ice initially got started when a thin layer of liquid (melt) froze in various spots. The ice that grew, first grew in the melt layer, then grew on top, essentially "sucking"** the vapor out of the surrounding air, thus drying out surrounding regions. This is why we see bare surfaces near the larger frost crystals. Those frost crystals grew from the vapor, just like snow, but are anchored to the surface because that's where the film froze. So, two types of crystallization are important: freezing of the melt (melt --> ice) then vapor deposition (vapor --> ice).
** The actual process is diffusion (the way perfume molecules reach our noses), but this term is a little more vivid.
The evanescent snow crystal
appears out of nowhere
The lines and boundaries
on its faces record a story
a story of a crystal's birth
a story of a crystal's life
But before the record vanishes
Who will hear its story?
A few years back, a correspondent of mine, Professor Akira Yamashita of Japan, long retired, sends me an email. In the email, he had a document with words and pictures of some small crystals that he'd captured back in the 70s. They were small crystals, essentially freshly "hatched eggs" from the frozen droplets upon from they had started. But some had small pockets of air near their corners.
To those who have studied any sort of crystal growth and have some familiarity with crystal-growth theory, these corner air pockets, or "bubbles", were in impossible locations. They should not be there. Pockets will form near face centers, not corners. But Prof. Yamashita also had a theory about their formation. His theory first looked sketchy to me, but I appreciate hearing about new ideas, so over the following years kept revisiting his theory, getting to think that it had merit, and wondering if it had other applications.
Then, just this past year, in our own ice-crystal experiments, we did something that apparently had never been done to small ice crystals in the lab before. We slowly grew a crystal in air. And we cycled it from slightly growing, to slightly sublimating (i.e., shrinking in size), to slightly growing again. A cycle that must happen in some regions of cloud. And here is what we saw:
After the sublimating, the subsequent growth kept a permanent record of the sublimation cycle in the form of 12 corner pockets, one pocket for each of the 12 corners of the crystal. These are pockets of air, just like the six large 'petal-shaped' pockets of air you see nearer the center of the crystal. They are forever stuck in the crystal. Stuck there until the crystal, with all of its features, vanishes back to air.
After seeing this, we ran a few more experiments, and each time we slowly grew, then sublimated, then grew again, we got corner pockets. The name 'corner pockets' refers to their location when they are formed; namely at the corners, next to the crystal perimeter. However, they remain essentially fixed in position as the crystal grows, and this means that as the crystal perimeter expands outward, the corner pockets will appear further within the crystal. Analogously, the 'center pockets' shown above formed at the face centers, on the crystal perimeter, back when the crystal was much smaller.
As to the theory of their formation, and how the theory can explain other observable features of snow crystals, you'll have to wait for a subsequent post.
In summer, the sun reaches higher elevations, bringing the possibility of new atmospheric displays. I saw this one in mid-June.
The top, upward arc is the more familiar and common 22-degree halo. But pay attention to the one on the bottom. Its colors look like a rainbow, but it has nothing to do with water drops of any sort.
It is the circumhorizontal arc, and it is as rare as it is beautiful. The colors are, in fact, more pure than those of the rainbow, and its origin is far more remarkable. It is remarkable because of the surprising set of conditions that must hold for it to exist. First of all, the particular region of cloud (at a certain angle to our eye) must consist mostly of ice crystals. Second, the crystals there must be nearly perfect tabular prisms. And third, these crystals must be in sufficiently non-turbulent air and of such a size that they fall in a nearly exact horizontal position:
After a few days of fine bright spring weather, the barometer falls and a south wind begins to blow. High clouds, fragile and feathery, rise out of the west, the sky gradually becomes milky white, made opalescent by veils of cirro-stratus. The sun seems to shine through ground glass, its outline no longer sharp, but merging into its surroundings. There is a peculiar, uncertain light over the landscape; I 'feel' that there must be a halo round the sun!
And as a rule, I am right.
The quote, from Minneart* describes a common ice-related atmospheric apparition. It appears in skies all over the world far more often than the rainbow, yet few notice it. As a graduate student, I read about halos and often looked for one, but didn't notice it myself until someone else pointed it out. As a post-doc in Boulder, I was out walking with Charles Knight, and I mentioned my lack of success. He glanced up near the sun, pointed, and said “why there's one right now”.
What I had missed in my readings had been the fact that most halos are rather indistinct and often incomplete circles. Indeed, now when I point out the most common one (the 22-degree halo) to someone nearby, they often don't see it. But occasionally, it is sharp enough (and colored) to the extent that anyone will see it if they bother to look up and glance toward the sun. And often it occurs with other ice-crystal apparitions that are even more obvious.
Last fall, while perched high on a rock face, belaying my partner up**, I saw such a vivid display.
The bright spot is called a “sun dog”, “mock sun”, or “parhelia”. They, one on either side of the sun, usually appear together with the 22 degree halo. Indeed, the sun dogs very nearly mark the spots where that halo intersects another arc called the parahelic circle. Their cause: horizontally oriented, tabular ice crystals.
Thomas H. Huxley once wrote the famous line:
The great tragedy of Science: the slaying of a beautiful hypothesis by an ugly fact.
Great man and a catchy phrase, but perhaps he was being a bit overdramatic. To me, the slaying of a “hypothesis” (i.e., pet theory or just idea, really) is itself a beautiful thing. It means that one can do a lot of damage with just a simple observation. I like it, even when I'm shooting down my own damn theory. Here's an example:
Some time ago in my experiments, I saw the following ice growth sequence:
What you see there is an extremely small, thin ice crystal growing from the tip of a glass capillary into air. (Size-wise, the glass capillary is about 5 micro-meters in diameter, or about 1/10th the thickness of the hair on your head.) I saw it happen several times. As the crystal grew, it developed the prism facets that generally define the hexagonal crystal shape. Other people had seen such rounded growth before, generally within a few degrees of zero (C), though in all cases, the crystal had been extremely thin. You can also see this thin, rounded (non-facetted) form in some hoar-frost formations.
The mystery here is why the disk grows without the prism facets for awhile. I never saw this with thicker crystals, so I formed a little theory. The theory involves the source of the water molecules to the curved region of crystal: some come from the vapor in the air and some wander over from the flat, non-growing crystal faces on front and back. In the 1960s, people had tried to measure this “wandering distance”, but never determined a consistent value. My theory predicted that once the crystal thickness exceeded about twice this distance, the curved edge would transition to flat, giving rise to the hexagon.
But then I looked closely at this case:
That sequence shows the side and front view. The crystal doesn't discernibly thicken when the flat prism facets appear. Zing! That theory shot down.
On to the next pet theory. So maybe the key factor is the diameter of the disk: One might argue that the curvature of the crystal surface must be below a certain value, which means a large enough diameter is needed, for the flat facets to appear. Or, the size of the resulting prism faces must be larger than that needed to have several surface steps (which help ensure flatness).
But then I look at this case:
In that case, I see both smaller and larger prism faces forming at the same time (same thing can be seen in the previous image). So, I guess the curvature or size of the resulting face is not the main factor. Zing!
Some researchers had observed slight bending of the prism facet above -2.0 C in equilibrium. They postulated a “roughening temperature” at -2.0 C. This might explain the rounded disc edge, but wait! 1) This disc edge becomes facet as it grows, so it is not merely temperature, and 2) these crystals are below -2.0 C.
Well, there are always “impurities” to blame! Crystal growers are fond of blaming some trace, active chemical, or “impurity” for inexplicable results, so we could theorize that the above show the effect of surface impurities. As the crystal grows, the area over which the impurities distribute increases, thus diluting their concentration, and thus reducing their effect. Yes! This could explain these results.
But, wait, what about this:
The above shows two sequences (same capillary) in which some prism facets have formed, but some remain round. In the right-side case, one corner even rounds as it grows. Hmm, not a likely result of impurities. Zing!
So, I am down to one last theory. I do not yet have the data to shoot it down. And I'm not telling you, or I'd ruin the fun. We just need more data.
All in all, I think Huxley needs a little tweaking to serve my view:
The great beauty of science: the slaying of a pet idea by a simple observation.
Morning on the spring equinox, the first day of spring, brought a few gifts from winter. I) Film-frost, accentuated with hoar-frost on the cars.
See the white of the hoar, following a pattern set down by the thin layer of film-frost. The roof of one car:
And the roof of another:
So it went. As I walked through our parking area, I saw a different pattern on every car - a pattern that told a tale of the night's weather and evening conditions. The film frost here is dramatic because the water film was thick before it froze. And it was thick because it had been raining in the previous evening. Later that evening, the skies cleared, and the temperature dropped rapidly, ensuring that the thick film would freeze. And the warm wet weather of the previous day left plenty of vapor to deposit as hoar-frost.
Car hoods had their own story to tell:
In early January, while visiting a cold, dry region, I saw this frost on a wooden fencepost.
The pattern resembled a cluster of butterflies. In the shade, these "butterflies" were blue, reflecting the blue sky. In the sun, they were bright white:
These are a type of hoar-frost, and though hoar-frost grows by the same processes as snow crystals, they can take on an even greater variety of forms. That they have more forms is a consequence of the fact that they can experience much greater levels of humidity, that is humidity relative to their temperature. This greater degree of humidity produces faster growth. Frost forms can also be more unusual because the proximity of the crystals alter the vapor gradients.
When water droplets land on ice, what happens?
If the ice is at least several degrees below zero (celcius), the drops freeze quickly, and build up a whitish, bumpy surface.
When the ice surface is heated to melting, the droplets vanish into the melt.
When the ice is instead very close to zero, the droplet spreads out, but not completely. You can still see the boundary of the droplet.
In the early 90s, there was some scientific debate about the ice surface near zero. Some said it had a thin, liquid-water surface, a layer that allowed us to ice-skate and make snowballs. Others said that if that were the case, then a droplet placed on the ice would spread out and vanish. Experiments showed the droplets didn't vanish. The jury is still out on the nature of the ice surface.
It had been a continued spell of cold days and nights, the ground snow-free, the air clear and dry. No film-frost on the cars in the morning, and no spikes of hoar frost sticking up on grass or post. The only signs of ice had been the frozen pond and the needle ice, making the ground crunchy underfoot.
And yet in the woods and lawn, hoar frost still lurked.