Snow is usually imaged in air, the single crystals laying flat on some substrate such as glass. The method is relatively simple, but one must work fast to image the crystal before it appreciably sublimates. Sublimation first rounds out the sharp edges and then causes the crystal to shrink. Generally, this sublimation happens because the photographer is radiating too much heat to the crystal. Conversely, particularly in very cold conditions, the photographer’s breath may deposit fog near the crystal, causing the crystal to grow.

Such issues vanish if one instead captures the snow in a cold fluid before taking the image. To work, this fluid should not dissolve the ice, be less dense than ice, be fluid enough to completely spread over the crystal surface, and be transparent. Other than preserving the crystal, the method has several other advantages. For example, in his laboratory experiments in Hokkaido, Japan, Tsuneya Takahashi lets the crystal fall into a cold suspension of two transparent, cold silicone oils. He sets it up so one fluid is denser than ice, one is lighter than ice, so the crystal falls through the light oil and rests on the (transparent) interface between the two fluids.

This method sounds complicated, so why use it? One, as the oils are immiscible with water, they block water molecules from arriving or leaving the crystal surfaces, so the ice crystal shape is preserved precisely for as long as the fluid is below 32 F (0 C). Two, after imaging the crystal, the fluid is warmed above melting such that the crystal melts into a spherical drop from which he can easily measure the volume and thus infer the mass of the original crystal. A third advantage, more difficult to exploit yet sometimes used, is that he can get top and side views of the same crystal. Other researchers in Japan have also used silicone oils to capture ice crystals in the lab, as well as naturally falling crystals, mainly for the first and third reason. They use just one oil type, a lighter oil. Charles Knight at NCAR in Boulder, Colorado had a fourth reason for using a cold fluid: better imaging. That is, one can image greater depth detail because light scattering off the surfaces is greatly reduced, particularly if the fluid is very clear and has an index of refraction close to that of ice. By reducing the scattering, one can see through surfaces to underlying surfaces. He would use gasoline or hexane fluid.

I don’t have a photomicrography setup to take detailed images of snow, and we rarely get snowfall with nice single crystals anyway, but I wondered how well the method might capture falling snowflakes. That is, could I at least see their rough shapes as they fell through the fluid?

Here, we typically get about one light snowfall per winter (2-3"). A relatively large snowfall happened this past weekend, depositing about seven inches. At first, the particles were small, probably highly rimed single crystals or small aggregates. Later, larger snow particles fell, and these particles were clearly snowflakes (i.e., aggregates). In preparation, the previous night I set out two covered wide-mount jars, one with water, the other with Coleman camping fuel (white gas), which has similar properties to gasoline. In the morning, the one with water had frozen, so I knew the other was also below 32 F.



Outdoors, I set the jar on top of my car, put a wooden chopstick in the jar both to focus on (my camera only has autofocus) and to provide a size reference, set a small LED light panel to the side for brighter illumination, and then opened the lid. The flakes fell into the fluid, and fell down to the bottom of the jar. They fell through the fluid slower than they fell through the air, but it was still too fast for me to see how well they were focused. Turned out that they were not very sharply focused, yet one can still see their general shape and fall orientation (below). In general, the flatter the flake, the more it tends to orient broadside to the fall direction.




Obvious improvements would be a better jar, such as one with a flat, smooth glass front, a better camera, and a thicker fluid to slow down the rate of fall.



Such improvements will have to wait at least until next winter.


--JN

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.

Read more »

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.

Read more »

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.

Read more »

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.

Read more »

Snow cleared from a city street slowly vanishes, leaving lines of "dirt".

(Click on images to view more closely.) Where the snow has a distinct edge, you can see the dark lines are ridges, some of which can be quite sharp:

Why is this?

Cryoconite ridging

This all happens because the snow is vanishing but the dirt is not. The vanishing is actually called "ablation", meaning some combination of melting, sublimating, and evaporating. It is mostly melting in the above case, but it is possible that evaporation helps to form the lines. About the dirt, it is not clear exactly what it is. The term "cryoconite" is used when similar dark dirt falls and clusters on glaciers and ice sheets, so to be specific, we might call it "cryoconite ridging".

Read more »

Last July, I attended the AMS (American Meteorological Society) 15th Conference on Cloud Physics, which was combined with a similar conference on atmospheric radiation. They have these about every four years, but I have missed all those going back to 1995. So, I had a lot to catch up on. Actually, by far the most enjoyable part of this conference was meeting old colleagues and making new acquaintances, including many who I had known only through their papers and email correspondence. As to why I waited so long, I suppose the main reasons were time and cost. Even without travel and lodging expenses, conferences are expensive. This one was $600, and the fee for the abstract I submitted was an additional $95. But in this case, the conference was only a 3-hr drive away (Vancouver, BC) and my co-worker very nicely picked up the conference fee and hotel tab. Also, this conference had a special session dedicated to the work of my former advisor, Prof. Marcia Baker. In that session, I presented the following poster.

I've described a little about corner pockets in a previous post, and showed a few of these panels before, but will briefly go over them here.

Read more »

This post is about the misrepresenting of snow crystals in public, not about misbehaving crystals and not about snow that has gotten dirty.

No doubt you've seen it, the Christmas card with four-pointed "snow" falling, or the sweater with an eight-pointed "snow" emblem. Once, at a holiday party, I saw such a sweater and remarked on it to the wearer. I was told that, with snow, "no two are alike", and apparently that was supposed to justify anything goes. Sorry, but it doesn't work that way. After all, we can say the same thing about people, that is, no two of us are exactly alike, and yet we do not regularly see sketches of people with five arms or three heads. That is because people do not come in these shapes. Similarly, snow-crystal growth allows unlimited crystal forms, but nevertheless follows strict rules. If you understand these rules, you too can point out impossible snow-crystal shapes. I give the rules below.



But to help illustrate these rules, I first present below examples of real and "good" snow, together with some examples of bad snow:

Click on this (and any image here) to see a larger version.

Read more »

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. 

 

-JN 

 

* 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. 

 

 

To understand snow formation, one must know a little about clouds. 

Q: What is in a cloud?

A: Air, dust, vapor, droplets, and often, ice. 

Q: How much air? How much liquid water? How much ice?

A: The answers will probably surprise you. See my short 20-min presentation below. I gave this recently to the Bellingham, WA Snow School. (23 slides, but due to file-upload-size restrictions, I had to put them into three parts below, 10 slides, 6 slides, 7 slides.)

Snow, rain, and weather affect everybody, yet how many of us learned in school even the most basic facts about precipitation in school?

Q: Who first realized how ice grew in a cloud?

As described in my presentation, he realized this by observing frost on the ground. 

Q: Who first realized how Alfred's theory was intimately connected with rainfall? 

Tor discovered this by observing fog in a mountain forest, and like Alfred, applied some of his physics knowledge. 

In my presentation, I discussed Alfred Wegener, the roles of the different cloud components, and briefly how the ice, once formed, takes on its strange shapes: 

 

First 10 slides (with blue text added to account for the things I said during the talk):

http://www.storyofsnow.com/media/blogs/a/Jan2017/snowschool_annotated1t10.pdf?mtime=1484585328

 

Next 6 slides:

http://www.storyofsnow.com/media/blogs/a/Jan2017/snowschool_annotated11t16.pdf?mtime=1484585328

 

Last 7 slides:

http://www.storyofsnow.com/media/blogs/a/Jan2017/snowschool_annotated17t23.pdf?mtime=1484585309

 

 

Later, I will show specifically how the ice gets arranged into all these strange shapes. 

- JN