Category: "Snow Science"

Caution!

February 1st, 2012

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

Snow on a Freshly Frozen Pond

January 15th, 2012

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

The Six-fold Nature of Snow

March 15th, 2011

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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Ice Forms on Slow-moving Water I: Caterpillars and Cellular Dendrites

January 15th, 2011


During my last winter in Japan (2009-2010), I would walk around a neighborhood park on frosty mornings, looking for interesting ice forms. It was in this park that I found one rock (only one!) that on some mornings would sprout hair-like ice, arising from liquid water within (see the last images in my “Ice on the Rocks” post, Jan 27). This park also has a small, slow-flowing brook that, despite the relatively warm conditions, often freezes over. Usually, upon freezing, it shows an ice pattern consisting of many long (~ one foot) straight lines – a typical pattern you usually see on glaciated puddles and ponds. But on the morning of February 4, at one spot right before the water tumbled over a waterfall, the ice surface looked slightly different. It had lines, but the lines were short, thick, and bumpy (see photo below), looking a bit like black-blue caterpillars scattered about.



From the small bridge spanning the brook, I reached down, pulled some ice out, and was shocked at what I saw. This was not “solid” ice, but rather a bunch of very thin ice plates loosely resting on each other – somewhat like a deck of cards spread out in a fan, though in this case, each ice plate had its own size and shape. Also, unlike other thin ice plates I’d seen before, these ones resembled neither the fast-growing ice dendrites (the snow-crystal-like, branched forms) nor the slow-growing ice discs. The photo below also shows that the plates were big – the section shown being several inches across.



Over larger streams and faster-moving water, ice can develop into something called frazil ice. I’ve no experience with frazil ice, never having lived in a cold-enough region, but I this caterpillar ice may be different. For one, the water was extremely slow-flowing. I could not discern the flow, even in ice-free regions, whereas the descriptions I’ve read about frazil ice involve easily discernable flow rates, flow rates that introduce turbulence, crystal collisions, and mixing below the surface. Also, the ice crystals I saw were much larger than the millimeter-sized pieces I see reported for frazil ice. So, the caterpillar form might be a transitional form, between frazil and that over stagnant water.

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Hoared Hail and Coraline Cups

December 31st, 2010

As far as I could tell, nobody had predicted hail last night, yet there it was on the ground, the largest hail I’ve ever seen here in the northwest. It fell on wet ground, freezing to the surface and then growing hoar columns, making the ground white and crunchy.

The parking lot in front of our apartment was covered in ice, mostly clear (i.e., "black") ice, yet, because of the ice lumps, it was not slippery under my boots. I kept expecting to slip, but never did. On the curb, where it was a little colder, the hoar was much thicker, looking like an inch of snow.

Same story for the tops of cars, where it had been coldest. The temperature was such that the hoar was mainly columnar. Columnar hoar, indeed all hoar, grows just like snow – when an excess of water molecules deposit from the vapor – and yet columnar hoar tends to develop more in the “cup” (C1b) and “scroll” (C1i) forms (see my Feb. 14 posting for the meaning of the symbols).

But unlike the pencil-like columnar forms of falling snow, these cups and scrolls grow broader at their growing end, even branching out into a pattern a bit like coral.

And if you look closely at the above shot, you will see that the “branches” grow along the same axis as the crystal from which they sprout. (For the above shot, I put a piece of black cloth in back to show the boundaries more clearly.) Such branching contrasts with that for tabular crystals, such as the ones on the familiar six-branched snow crystal.

Why do the columns widen at their growing ends, thus making a cup-shape, why do the cup rims sometimes curl into scrolls, and why do they branch out? I suspect the reason for all three (though not a complete reason) is that the humidity next to the growing crystals is very high – higher than that surrounding the typical column form that drops from the sky. Unlike the latter case, where the humidity is too low for the prismatic faces to grow outward, at least at any rate near that of the basal, here both the basal and prismatic faces grow at comparable rates. Such growth behavior also depends on the temperature, for reasons that still elude us.

- JN

More Tales of Mystery and Observation

December 1st, 2010

When I stepped out early Saturday morning, the air seemed relatively warm, particularly compared to the cold snap we had last week. Indeed, it was much warmer, and yet the parking lot in our apartment complex had a glaze that was much more dangerous than that during the cold snap.

But it was the frost that I noticed. Of course, the air was relatively warm, but the clear sky cooled the surface. Recently, snow melted, leaving plenty of open water to evaporate – perfect conditions for film frost and hoar.

I saw “striped-tail” film frost(1) on two car roofs, both times on the sunroof glass. Why only on the glass? Perhaps the glass, being a poorer conductor of heat, had a lower surface temperature. This lower temperature would have produced a thicker film of water, and the thickness of the film seems to influence the pattern. The bigger mystery though is the cause of the pattern. In a short online article(2) last year, I suggested a cause for the stripes on each tail, though I can offer no explanation for either the nearly uniform width of the tails or the meandering. In the case below, the pattern is dense with lazily wandering striped tails. When I look at it, I think of seaweed.
The metal car surfaces often had nice curving film-frost with dense hoar, while others had or condensed droplets with more isolated, spiky hoar. The latter appeared on my own car.

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The Window of Many Cacti

February 14th, 2010
It’s been two weeks since our last frost, and judging from my first dozen shots, it seemed like my photography skills dropped from mediocre to downright pathetic. But then I got a few good shots of frost on windows. The fact that I shoot windows on cars means that I see some special forms that would not normally appear on house windows. The most common type I call ‘cactus frost’ because of its resemblance to saguaro cacti.



The resemblance may be a bit more obvious in the following shot, which I took two years ago on seeing this form for the first time.

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How to classify snow crystals

February 14th, 2010

According to Edward LaChapelle’s Field Guide to Snow Crystals (1969), the most widely used classification for falling ice is a system of 10 types from the International Commission on Snow and Ice, which came out in 1951. A few years later, ice researcher Ukichiro Nakaya published a new system with four times the number of ice types. The number doubled 12 years later in 1966 when Magono and Lee published an extension of Nakaya’s system. Of all systems, I think Magono and Lee’s classification of 80 types gets the most use nowadays, so I show it here to help make more people aware of it.

To classify a crystal, you simply look at the table below and find the example that most nearly resembles your crystal. This table is a scanned copy of the one in LaChapelle’s book, with some markings cleaned up and a little color added. Click on it to enlarge.

For example, the crystal in Mark's Feb. 14 posting is P1c, a broad-branch crystal.

The number 80 seems large, but even this system has notable omissions. Magono and Lee include graupel (R4a, R4b), which are crystals that collided with many supercooled droplets, but they exclude crystals that collided with and adhered to other crystals. That is, they exclude snowflakes. Other omissions include distinct forms like the spearhead and seagull types as well as sub-divisions of forms they do include; for example, plates like P1a except with unequal sides such as the oft-sited trigonal forms (alternating prism faces are shorter/longer), and hollow columns like C1f except with hollows on all faces. And although they include 12-branched crystals, they don’t include 18 and 24-branched forms. They probably had a good reason for excluding these forms, but it would be nice to have a more complete table for the avid naturalist to refer to.

Choji Magono was a professor of geophysics at Hokkaido University in Sapporo Japan, and C. W. Lee was his student. I’ve known about other work by Magono for some time, but I’ve never found any other publications by Lee. I recently asked about Lee to someone who once was a student there, and he told me that Lee returned to North Korea soon after getting the PhD, apparently never to be heard from again. It sounds like a sad ending for a once very promising young researcher. If you can read this Lee, here’s a big thanks for your great dissertation work.

- JN

I am a giant snow crystal, imperfect thing of habit, bouncing along life's gusts,...

February 6th, 2010

Some reviewers of The Story of Snow mistakenly call me the main author. I don't know why they make that mistake - Mark's role as the main author is clearly implied by his copyright  on the dedications page and elsewhere. My role was just to check the text and make some suggestions.

But I am very glad he asked me to help. In fact, in 2004 I pitched an idea to some publishers that was similar in some respects to, though not as good as, The Story of Snow.  My version was fiction. In it, a boy in his yard kicks up a bit of leaf dust, and we track the dust rising through a cloud, eventually to return to the boy's yard as a snow crystal. One publisher expressed some interest, asking me to turn it into non-fiction. I didn't bother.

Another way that Mark and I have similar ideas in that we both liken snow to people. In The Story of Snow, the uniqueness of each crystal is likened to our own uniqueness. I have thought about other ways snow resembles people; and in my background write-up for the Junior Library Guild, I mentioned some of them. Inspired by some of the nice science essays I read in the New York Times, I developed these resemblances into a little essay and submitted it to them. It didn't make the cut, but I think you might enjoy it. Here it is.

 

Seeing Ourselves in a Flake of Snow

 

The silently falling snow crystal can vanish in one’s breath, yet can build up into glaciers that carve mountains. Individually weak yet collectively strong; people are like that too. And if we look deeper into the little flake’s tumble from the clouds, we may see even more parallels to ourselves.

Consider its birth. A snow-crystal’s origin lies with some fleck of mineral or organic matter kicked up from the ground. Though most never get far, the occasional bit of dust floats high enough to be caught in a series of updrafts. Upward it floats, cooling all the while, until dew nucleates on its surface. The dust, like the irritant in an oyster that starts the pearl, becomes a miniscule speck, engulfed in a tiny yet growing ball of water – a droplet. This droplet may be as near to a perfect sphere as we are likely to find anywhere. But spherical perfection doesn’t last.

When the droplet cools below freezing, it enters a supercooled state in which it may crystallize into ice. But it cannot freeze right away. Think of the supercooled droplet as an unfertilized egg, needing a tiny ice ‘seed’ to transform into ice. These seeds, properly called 'ice nuclei',

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Ripples

February 5th, 2010
Ripples in still water
When there is no pebble tossed
No wind to blow

--Grateful Dead “Ripple”


I don’t know where to start on this one. For some time I’ve been seeing concentric circular patterns on car windshields and car bodies – bands of white spreading out from a central point like ripples in a pond from a tossed pebble. Typically, they spread outward 3-10 inches or so before meeting up with ripples originating from another spot.



When I try to zoom in on the individual crystals, I usually see only vague outlines with the occasional recognizable form. What could cause this pattern? Here's a clue.

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