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.

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                Bending bending bending
                The fir branches are bending
                They are waiting for more snow




The famous Japanese poet Basho wrote a poem like the above, except it was about bamboo bending under the snow load.

I was reminded of Basho's poem yesterday, as we just had our first snowfall of the season. Here in Redmond, about 2 inches, just enough to bend some of the fir branches.

And not only the branches were bending.

I took the opportunity to check out the pond. It had an ice covering before the snow, but with the snow and warming trend, the ice had thinned, and gotten pressed down under the snow load. The glaze over the pond was bending. Where a hole appeared in the ice, water got pushed up and out, flowing in dark channels over the icy glaze, forming a spider-like or branch-like pattern in the now slushy snow. I climbed up a tree on the shore and took a few shots.

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When I last wrote about the classification of falling snow and ice (02/14/2010), I discussed the 1966 Magono-Lee system. At the time, this system was the most recent one, and as such, the one generally used in meteorology. And to those who wondered how many types of icy precipitation exist, Magono and Lee would tell you 80 types.

Although 80 sounds like a lot, Magono and Lee did not include at least one very common type as well as a few other interesting types. This major omission was snow-crystal aggregates, generally known as “snowflakes” by meteorologists. These are individual snow crystals that have stuck together during their fall and arrive as large open clusters of crystals, often with tens–to–hundreds of crystals in a big, round blob. (To meteorologists, a snow crystal is not a snowflake; rather, a snowflake consists of many snow crystals. It is like the difference between a trundled rock and a landslide.) In fact, the snowflake is probably the most common type of snow precipitation in most areas that receive snow. Several other omissions of the Magono-Lee that I mentioned in my 2010 posting included spearheads, seagulls, and the both the 18- and 24-branched forms.

If you were waiting for someone to fix the classification, well wait no more: we now have a new classification scheme that includes all of these and more. The new system has not one, but three types of snow-crystal aggregate (snowflakes), which are given the symbol “A” (for aggregate). There is also four types of spearhead crystal (symbol “CP8”), five types of seagull crystal (symbol “CP9”), and both the 18- and 24-branched crystals (P5e and P5f). In all, the new system has added 41 new types. In one big graphic, here is the new system:



The architects of this new classification scheme published the above table in summer 2013*. There were four authors (K. Kikuchi, T. Kameda, K. Higuchi, and A. Yamashita), making it harder to name the system after the originators, but hopefully their names will become known, just as Magono and Lee’s have.

I will discuss various aspects of this new system in subsequent postings. But for now, I point out that the number of basic types has increased over the years, from 10, to 40, to 80, and now 121. When will it ever end?

Of course, it may just keep increasing. In fact, if we plot the numbers vs the years, and look at the pattern, we might conclude “never”.

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While riding my bike home the other day, I saw what appeared to be a patch of light snow.


It was the only such patch around. Looking closer, I could see that it consisted not of snow, but of chunks of partly melted rime deposits. (Note how the pieces are long and narrow, like little icicles.)


Just above were the branches of a huge Douglas Fir tree. Perhaps there had been an over-active squirrel or two up there. Other trees still had their rime. Hard to see why only this tree would have its rime fallen off.

- JN

Two mornings ago, I saw this on the windshield of a parked car.


The bulls-eye pattern wasn’t centered on any particular feature on the windshield, and there were similar, though less developed, patterns nearby. See them on the photo below.


The dark parts are largely frost-free regions, and thus are regions that dried out during the crystallization event. (It was still dark when I took the shots.) Later, under a brighter sky, I saw a different pattern on the hood of another car, a pattern that I figure has a similar cause. In the hood case, shown below, the dry regions are bright due to reflection of the sky.


Now, about those concentric rings …

I puzzled over a larger such pattern in the Feb. 5, 2010 posting “Ripples”. The causative process that I proposed back then seems consistent with this newer observation, but I will clarify it here.

Before the first frost formed, the windshield, though seemingly dry, nevertheless had a thin layer of liquid water. This thin film had cooled to some temperature below freezing. (See the sketch “How hoar frost forms” in the Jan. 11, 2012 posting.) As the windshield and water film continued to cool, freezing was inevitable; the only issue was where. The first such spot to freeze must have had some feature, however minute, that was advantageous to freezing. It may have been a nucleant particle (e.g., a mineral dust grain, a type of bacteria, or even a tiny fleck of ice that wafted in from somewhere else), a slightly cooler spot, or a slightly thicker water film (e.g., from a scratch or indentation). But for whatever reason, the ice formed there first.

Now suppose the ice spreads outward from the nucleation spot in all directions with about the same rate. I suspect this happens when the film is extremely thin, as it would have been under the relatively dry conditions of this day. So, the frozen film grows outward, roughly as a disc. See the sketch below; the black dot in the center is the first place that froze.

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The "Ice Man" -- that's how a newspaper header referred to me after I gave a recent conference lecture:

A link to the article is here:
http://www.sctimes.com/article/20121023/NEWS01/310230011/Featured-speaker-details-snow-crystals-SCSU-storm-conference

It was a very enjoyable visit to the 7th annual Northern Plains Winter Storms Conference on the campus of St. Cloud State University in St. Cloud, Minn. We had various talks including one about forecasting a storm, the statistics of the snow-to-liquid equivalent ratio (e.g., in some areas about 13" of snow will melt to 1" of water, but regions and storms vary considerably, some being over 70 to 1),and one talk about how a late-season snowstorm might end a locust plague (unlikely, according to the speaker).

My talk described how a simple principle allows us to understand how a wide variety of snow crystal forms originate.

Here's the narrated talk. To view, click the image, then enlarge to full screen:

">
Part 1 (10 min):
-> Why study snow crystal shapes?
-> Some history about snow science.
-> The habit map.
-> Questions that will be answered in the rest of the talk.


">
Part 2 (13.5 min):
-> Basics of snow crystal science.
-> Why dendrites grow so thin.
-> Why needles and columns sometimes form.


">
Part 3 (6 min):
-> How the crystals get their branches.
-> Why they are six-fold symmetric.


">
Part 4 (14.5 min):
-> How the crystals get sidebranches.
-> Common errors we make when drawing snow.
-> Why they have so much variety.
-> Mysteries about snow.
-> What are the "messages in water".



It is a scientific talk, so it involves some diagrams and technical terms. But this one is pretty easy. Perhaps the only technical terms are "vapor deposition" and "supersaturation". Vapor deposition happens when water molecules in the air (i.e. water vapor) crystallize onto something, like a snow crystal or hoarfrost. The vapor must be "super" saturated for this to happen. Greater supersaturation means greater vapor density and thus faster growth. The above link goes to the first segment, and from there you can click on the subsequent segments.

On the flight home, I saw a subsun on the clouds below. A subsun is a reflection of the sun from tiny, flat, hovering plate-like (tabular) ice crystals. They are essentially hovering like microscopic flying saucers.



In the photo above, you can see the sun's reflection off the wing on top. The smaller reflection on the clouds below is the subsun. I used to think the subsun was rare, but apparently I simply wasn't looking. This subsun was there in various forms for at least 2/3rds of the flight. I've seen them on most previous flights. More on subsuns in the next post.

- Jon


Here's the abstract to the talk:
snow crystal seminar abstract.pdf

And here are the four parts of the talk in pdf form (from the PowerPoint slides):
snow order and mystery - 1of 4.pdf
snow order and mystery - 2 of 4.pdf
snow order and mystery - 3 of 4.pdf
snow order and mystery - 4 of 4.pdf

For a few years now, myself and another ice-researcher have been trying to get a research grant from the National Science Foundation to study snow. Well, we just recently heard that the funding came through and I’ll be starting on the project in March. Yeah!

You might wonder what it is that we don’t already know about snow. Well, the better question is, what do we really know? The answer is “not much”. There’s been a lot of experiments in the lab, spanning nearly 100 years now, with many interesting results, but nothing really clear has emerged. Here’s one of the basic problems: In a typical cloud consisting of supercooled droplets and snow crystals, we can calculate the crystal’s rate of growth fairly accurately if we know the crystal shape. But it is extremely difficult to predict the crystal’s shape, and the rate of growth can change a lot with the shape. That’s the thing about snow crystals - they come in a bewildering variety of shapes, and we still have no clue as to why. And if the crystal isn’t surrounded by supercooled droplets, the situation is even worse.

We argued in our proposal that the main reason we don’t have a clear picture about the development of snow crystal shape (and even growth rate) is because of problems with the experimental methods. Instrumental influences easily creep into the experiment and alter the crystal shape. We suggested a new method based on one I helped developed in 1994-1996. In that method, we grew single ice crystals from the tip of an ultrafine glass capillary (about 10x thinner than the typical hair on a person’s head). Some of the images below show what I mean about shape variety.


The crystals are all very small and compact because we were interested in growth with very low humidity. In the series (a1) - (a7), you see the crystal growing as I rotated the capillary to see all of the crystal faces. Sometimes I let a crystal grow for a week or more.


You can see that sometimes a crystal is 5-sided. But the angles between the prismatic faces are always multiples of 60 degrees. I don't think I'll ever see a 5-sided snow crystal that looks exactly like a pentagon.


The large crystal at the bottom left & middle was starting to melt. The melting formed cusps on the edges of the face. The one at the top right is called a bullet rosette. These crystals are very common in cirrus clouds and other very low-temperature clouds. The sequence at the upper left shows something we can do with a capillary: we can suck the crystal out from the inside. In that case, I put a vacuum on the other end of the capillary and a void started to form. Then, inexplicably, a crystal started growing inside the void! You never know what will happen in these experiments.

By this time next year, I hope to be able to show new crystals we grew. And sometime soon after that, have some reliable results.

-- Jon

Hoar. It's just a white coating on things, so why does it make everything look more interesting?

I saw this hoar coating on a plastic trash-can lid:


The hoar frost on the lid had various whirls, just like I've seen on the plastic surfaces of car door handles and side-view mirrors. This hoar was a little different though in that the crystals were definitely sticking up and not laying flat on the surface. Nevertheless, the fact that they show a pattern at all, and are not just randomly oriented, means that there must have been a liquid film of water that first froze to the surface. The film froze, producing a pattern of crystal orientations on the surface, and these orientations were not revealed until the hoar frost grew. Hurray for hoar!

Here's another warning:


The hoar crystals are longer on the raised lettering, particularly near edges. This is not because such places are further from the ground, but because they have more radiative cooling (due to their more expansive view of the sky) and can stick out into regions with a greater density of water vapor molecules.

If you click on the images, you can see the crystals a little better. But I forgot my tripod on this particular morning (I took the shots after I got to the office), and so the images aren't as crisp as my other close-up shots.

- Jon

Back when I was doing post-doctoral work in Boulder, Colorado, Charlie Knight, the head of my lab, introduced me to strange ice phenomena. The most memorable one happened after the weather had been sub-zero for a few days and then we got some snow. When this happened, we stopped work and drove out to some shallow ponds to look at the patterns on the surface. Sometimes odd, concentric circles formed.


To see the size of the rings, check out the overview.


The guy in the background is Charlie. He is sawing through the ice to get a sample. Behind him are two visitors who came out with us that day. The ice was about 2" thick, if I remember right, and would make some cracking noises sometimes as we walked on it.

I don't know if he figured out the cause of the pattern. I never made any progress in understanding it. Anyway, what seems to happen is that water gets pushed out through a small hole in the ice, and the water apparently spreads out in a circular region. But why does the lightness of the ice change in nearly equally spaced, discrete steps? And why is it whitest in the center?

I figured that if water flooded over the ice in discrete steps (day-night temperature fluctuations, as we think happens with the pancake ice?), then the region in the center would be the darkest, not the whitest. For example look at this counterexample.


This shows the hole where water comes out, but the water floods outward in a ragged fashion, not like a concentric circle. This type of flow has been studied a lot in the laboratory and has the technical name "viscous fingering". Anyway, notice that the region near the center is darker, not whiter.

Any ideas about the concentric circles?

--Jon

Some people have asked me why snow crystals have six corners. The answers I’ve seen in books and the Internet are incomplete at best. Here’s a more complete answer. (A pdf copy of this article is here.)

How the crystal got its “six”

It’s a cold winter’s day in Prague, late in the year 1611. A man walking home is worried because he has no new-year’s gift for his friend, benefactor, and fellow philosopher. Upon crossing over the Charles Bridge, some snow crystals quietly land on his coat. The crystals at first distract his thoughts, and then bring great delight. Of course! The six-cornered crystals on his coat have brought a philosophical puzzle to present to his friend: Why, he asks, do they always fall with six corners, not five or seven? What is the origin of the number six? Who shaped the little head before it fell, giving it six frozen horns?

Indeed, what is the origin of the six? Why do snow crystals often look nearly the same when rotated by 1/6th of a turn?

Johannes Kepler, like others in his time, did not have the atomic theory of matter to use as an explanation. But he was nevertheless on the right track when he later wondered whether ice bore a relation to the honeycomb of a beehive(1). Indeed, the honeycomb structure is very similar to what we see in the internal molecular lattice of a snow crystal. If you could zoom in about a million times into any region of a snow crystal, such as the branch tip in figure 1A below, you would see a lattice of hexagonal rings, as sketched in B. Each ring of six oxygen atoms (black) has the six-fold symmetry; that is, the ring looks the same when rotated by 1/6 of a turn. This six-fold symmetric pattern inside ice provides a simple, though rather superficial, answer to the origin of the “6”.

But things are not so simple. If you examine the rings further, you will notice that their sides are all rotated by 30º in relation to the sides of the crystal. Why the 30º twist? Moreover, if you look at the orientations of the molecules (i.e., the red hydrogens), you will see that the rings are actually not six-fold symmetric. And, upon turning the ring on its side, as in C), notice that it’s not even flat. So, at the very least, the simple answer is incomplete.

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