[This is the fifth in the series of re-posted articles, from 2010.]

The most comprehensive study of frost and small ice formations was published way back in 1907 by Wilson A. Bentley. His article was split into five successive issues of the Monthly Weather Review (1) with the title “Studies of Frost and Ice Crystals”. In the article, Bentley details his classification system for frost and some related ice forms, and gives examples of nearly every type – in total 274 examples in photographs. His classification system is ingenious, the first (and probably only) classification system for frost and ice on surfaces. Unfortunately for such an extensive, pioneering article, few books or articles reference it.

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In 2012, I gave a "science cafe" talk with a local series sponsored by the Pacific Science Center, KCTS public television, and Science on Tap. The title was "The curious world of ice and snow". The location was a bar in Kirkland, but open to all ages. When I showed up with my family, they tried to seat us in the backup room, the regular room having filled up, but I said "Oh, well I'm the speaker" and they kindly created a space for my family in the regular room. I was indeed surprised at the crowd. People are apparently more interested in ice than I thought. (Hmm, but where are they when I post here?)

Click on any image to see an enlargement.

The basic structure of each talk was to give a lecture of about 30 minutes and then allow up to an hour (I think) for the Q&A. In my excitement, I had created 41 slides, in retrospect too many for the allotted time.

Given all the time spent preparing the slides, I hope that by posting them here that even more folks can enjoy the images and discussions. But, instead of unloading all of them on you at once, I will break the discussion into three sections. By adding the following table of contents, each section will have 14 new slides and the total will be 42, which according to Douglas Adams* is a really special number.


The contents of this section is part "1", written in green font.

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Last October, I gave a talk at the University of Washington about our recent experiments and ideas about snow-crystal growth. My pitch was general and short, as few folks work in this area and I'd hate to bore them with a long lecture. So, I was delighted to see quite a few graduate students in the audience, some of them asking good questions.

Instead of giving the narrated presentation here as a video, I will give the slides (23) with brief explanations similar to what was spoken at the talk. Narration below each slide. Skip to the ones that look interesting, and click on them to enlarge.

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If you inspect the tip of a growing icicle, you might be surprised to find it hollow. Skeptical? Well, if you think the conditions suitable for icicles outside, put a toothpick in your pocket and go outside. Then when you see a likely candidate, poke its tip with the toothpick.

Click on the image to enlarge.

Not all icicles are growing, of course. If the supply of flowing water has dried up, the tip may be solid, or if the air has gotten warm, then the tip may be rounded and wet. The pic below shows a longer, but solid-tipped, icicle next to the growing one. Its melt-water source is too low for the tip to grow.


Transitional situations occur as well, such as the icicle that just started to melt and still has a hollow tip, or the completely solid icicle that just starts growing again and has yet to develop a discernable hollow. The drip may also be intermittent. You might not find that growing icicle right away, but keep that toothpick handy.


As to why a growing tip is hollow, I won't attempt to prove it here, just try to make it seem plausible. Spend a few moments considering the following two simpler situations that show the basic ideas. You can even apply these ideas to other forms involving freezing water...

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The hoar-frost crystal shoots up like a thin, solid straw, then suddenly opens up into a cup-like shape. I have seen it often enough to give it a name: "martini hoar".

The cup can be weirdly segmented and polyhedral, but it nevertheless widens suddenly. Here are a few more (Sorry for poor photos—someday, I hope, I'll get better about photography.)




Here is a larger view of the region. Note the similar hoar coming down from the top, but without a clear view of the base.



This sudden widening feature has bothered me for awhile, but I was delighted the other day to figure out a plausible reason. My delight was made even greater because the reason involved measurements I made in the lab two decades back. The measurements were to understand snow-crystal habit, but apply equally well to hoar frost because hoar grows just like snow except it is attached to the ground.

Now that I have viewed some of the older pictures I took, the actual growth phenomenon looks more complicated in terms of crystal shape, so I am not so sure my reasoning explains things so simply. Nevertheless, it should apply well to many cases, and at least is worth learning because it involves important growth principles that also apply to snow.

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After a long period of work (on and off), we have an accepted paper on the corner pockets we discovered (see here). But during the writing stage, I thought about collecting earlier ideas I had developed during my correspondence with Prof. Akira Yamashita in Japan, and as a result, the paper ballooned. Originally, it was to be a short note about the pockets, but in the end, the central theme was instead the new notion of the lateral (or sideways) growth of crystal facets. Actually, one type of lateral growth had been long known, sometimes called "facet spreading", but we collected together three types of lateral growth and describe how two of them are particularly useful for explaining a wide range of observed features on snow and ice crystals.

But before describing some of our findings, recall that all science publishing costs money. Almost always the research grant pays the page charges. However, our grant ran out three years ago, and this paper is particularly expensive (estimate: $3600) because we needed many pages to argue our points and show how lateral growth can explain many growth forms. If you can contribute, we will gladly send a signed copy of the paper acknowledging your help. Or, if you have just enjoyed some of the articles on this blog and would like to help me continue, you can contribute here as well. Here is the link:

https://www.gofundme.com/f/publish-a-research-paper-on-snowcrystal-formation

The paper in its submission format is 58 pages: (as always click on the image to enlarge)

The published format will reduce the length a little (mainly shrinking the figures), but it will still be much longer than the standard 10-15 page paper.



One key figure to help explain some of these "lateral" concepts appear in Fig. 2, reproduced below. This figure shows a just-frozen droplet, which is often called a "droxtal". Crystal facets have appeared for eight faces, shown as the shaded flat regions in the top sketches.

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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, catchy phrase, but perhaps a bit overdramatic. To me, the slaying of a “hypothesis” (i.e., pet theory or just idea, really) can be the 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 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.
Zing again!

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.

-JN