Some reviewers of The Story of Snow mistakenly call me the main author. I don't know why they make that mistake; after all, that Mark is 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 in still water

When there is no pebble tossed

Nor 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. In the image below, you can see a moat in the upper right corner, so I think this pattern occurs when there are many tiny droplets condensed on the surface. The dark parts between ripple crests may be imperfect moats.

The first idea I had was this. A fog of tiny droplets condense on the surface. But one spot is colder than other spots. A group of droplets freeze in this spot, and then grow from the vapor, thus sucking vapor out of the air. This creates a roughly circular dark moat around the relatively white crystals in the center. Slightly beyond the moat, where it is still colder than other regions, more droplets start to freeze, thus creating the second ring. The pattern repeats, producing concentric ring after concentric ring.

But there is one big problem with this idea. The center is dark, not white.

Anybody got any better ideas?

- JN

I’ve seen this whirlpool pattern on two mornings on the same plastic side-mirrors of the same car.

The hollow columnar crystals are oriented lengthwise along concentric circles, which strongly suggests that an underlying film froze with the same rotating crystal orientation. This is strange. To see why I think it strange, we need to specify crystal direction. Consider the ice-crystal optic axis, the length-wise direction of the columns (or the direction straight into a stellar-star crystal). If we draw the optic axis for each crystal as an arrow, then we would have something like the following picture.

When a film of water on a smooth surface like glass or a car roof freezes, the preferred crystal orientation is that with the arrow pointing straight up and out of the surface. So, I find the above pattern mysterious - why don't the arrows have any trend toward pointing upward? Why do all  the arrows  stay in the same plane? Another mystery is the fact that I’ve seen this same whirlpool pattern with about the same center spot on both side mirrors on more than one morning. Perhaps the whirlpool pattern arises somehow because the surface is curved. Or maybe films of water on plastic freeze differently than films on smooth metal or glass. For now, I’ll call this the mystery of whirlpool hollows.

- JN

It often seems like people refer to any kind of ice stuck on something as frost. If one looks in books or the Internet, one can usually find a specific term for the many different and interesting ice formations, but a term used by one group of people generally differs from that used by another. Wouldn’t it be nice for scientists and naturalists (at least) to have some generally universal, agreed-upon way to describe any surface-ice formation? Snow crystals have such a system, so why not surface-ice forms?

We could try to establish word-names for each formation, like “frost flower”, but different people are used to using different names for the same thing, and they are unlikely to give up the habit, particularly if they are unfamiliar with the language of the term’s origin. For example, there are several well-used terms for the ice columns in dirt that I discuss in “Ice on the Rocks” including “frost heave”, “needle ice”, "ice columns", and “pipkrake”. But “needles” also names a type of snow. Another example: The white, curly ice hairs that can extrude from plant stems and logs is sometimes called “frost”, “frost flowers”, “Ice flowers”, "needle ice", and “sap crystals”. But the two flower terms are also used for several other, very different things. Though a few terms seem to be used consistently in English, like “hoarfrost” and “icicle”, most aren’t. And probably none work across all languages. So, instead of using word-names, a set of symbols may work best.

Wilson Bentley gave a good start to such a universal code for surface ice way back in 1907. His article “Studies of frost and ice crystals” gave an ingenious method for classifying several types of surface ice [1]. The article is a fantastic source of information about hoarfrost, window-pane frost, and ice that grows in bodies of water like lakes and rivers. Unfortunately, I know of no one except Bentley who ever used his system. This lack of use is probably due to issues unrelated to his system’s merits, so  his system might need only a little revision and some promotion to get it into use. My purpose here is to suggest such a revision.

Bentley’s system used three letters to classify many kinds of ice-forms:

Position 1: capital letter giving the kind of frost or place of deposition.

Position 2: capital letter giving the characteristic form of the ice.

Position 3: capital letter giving how common the ice form is.

Position one could be "W" for window frost, "H" for hoarfrost, "I" for window-ice, "M" for massive ice (e.g., ice in puddles, lakes, and rivers),  and "S" for hailstones. For short, we could call it the WHIMS system. As an example, the following ice formation is “IFA” in his system.

Although the “I” stands for window-ice, ice on smooth black metal, as in the above case, has similar growth patterns. The second letter “F” stands for feather-like, and the “A” means that it is among the most common types of I-forms. The ‘striped tail’ part is mine; I called this the striped-tail formation before I learned of Bentley’s system. Though his system is a great start, I see some problems. One, the first letter seems to ignore the forms of frost that grow on other surfaces that do not produce patterns like those on glass (e.g., rough plastic). Two, his distinction between window-frost (W) and window-ice (I) doesn’t seem right; I think most examples he gave for W can be lumped with I, being examples of ice that formed in a film of liquid water. Three, his system doesn’t include many types of ice, such as ground ice and doesn’t distinguish ice that formed from a succession of sources with ice that formed from only one source. Moreover, his choice of second letters may cause some confusion because one letter may represent several different characteristic forms; for example, the “F” in “IFA” stands for feather-like, but F can also stand for fiber-like, filament-like, and flower-like. Another unclear point about his second letter is that ‘the characteristic form’ may refer to the shape of the individual crystals or the shape of the overall pattern. I think these two cases should be separated.

The third letter in the WHIMS system, being about how common a type is, seems biased to observations Bentley made at his farm. However, we could just as easily view the third letter as representing the order in which the form was named. For example, if the last letter is “D”, then we can understand this as meaning that the ice form was named after the “C” form. The nice thing about this is that we can keep adding new forms as they are discovered.

Later, Ukichiro Nakaya published a symbolic system for classifying falling snow [2]. His system appears similar to Bentley’s, but instead of using all capital letters, only the first position has a capital letter. The second position has a number, and the third a small letter. For example, most of the snow crystals shown on this blog by Mark are P1d, P1f, and P1g. The first letter gives the main characteristic of the crystal, with most being either “P” for plate-like (i.e., tabular) or “C” for columnar. A nice feature of Nakaya’s system is that the symbol shows when the main characteristic changes; for example, “CP1a” is a crystal that began as a columnar type (C), and then P1a crystals grew on the two ends of the column.

So, considering the good and bad points of these two systems, here is my idea for a revised system. I call the system “BEDFISH” for a reason mentioned below. In this system, a name for any ice type consists of four positions.

Position 1: a capital letter designating the main source of water of the ice.

Position 2: lower-case letter or letters designating the overall pattern or size of the ice formation.

Position 3: a capital letter to characterize the shape of the individual crystals.

Position 4: number, starting from 1, indicating either how common the form of this water source (or sources) or when it was named.

I came up with seven symbols for the first position or source of water:

B: from bulk liquid, meaning puddle-size or larger.

E: from extruded water; for example, soil moisture or plant sap.

D: from droplets, condensed from vapor or deposited as a droplet.

F: from a film, thick or thin, and including water in a narrow channel.

I: isolated crystals formed on a surface but from the vapor.

S: from snow crystals lightly deposited on a surface.

H: hoar frost, meaning crystals similar to I, but densely packed.

Hence the name BEDFISH. These symbols cover all the cases I thought of, and may be sufficiently general to cover other cases. Bentley’s system did not include forms that fit into E and D. For example, WHIMS did not include rime, large frozen droplets on a surface, icicles, ice stalagmites, and other ice forms that fit here in group D. Icicles don’t perfectly fit into any of the seven categories, but is most similar to other types in D.

An example:

HuT1 = hoarfrost (H), uniformly spread over a surface (u), consisting of tabular crystals (T) of type 1.

For this system to cover as many cases as possible, there should be a way to designate successive sources of water. A simple way combines symbols using a slash “/” as a separator, with the most recent source first. For example, HcC2/Fg3 could represent hoarfrost (H) with a “combed” look (c), made of columnar crystals (C) of type 1, but clearly growing from a frozen film (F) with a grainy (g) pattern type 3. For this case I assumed that the shape of the crystals in the frozen film is unknown, and this explains the lack of a second capital letter in the F part of the name. (This is just a possible case – I haven’t settled on these meanings for “c” and “g”. Besides, maybe this should be "u" for uniform, instead of "c" because the combed look comes from the Fg3.)

 

Let me know if you have suggestions.

- JN

References

[1] Bentley, W. A. (1907) Studies of Frost and Ice Crystals. Monthly Weather Review. This was published in five successive issues, from August through December. Each part is available freely online, with the last part (e, below) having 274 crystal images.

a) http://docs.lib.noaa.gov/rescue/mwr/035/mwr-035-08-0348.pdf

b) http://docs.lib.noaa.gov/rescue/mwr/035/mwr-035-09-0397b.pdf

c) http://docs.lib.noaa.gov/rescue/mwr/035/mwr-035-10-0439.pdf

d) http://docs.lib.noaa.gov/rescue/mwr/035/mwr-035-11-0512b.pdf

e) http://docs.lib.noaa.gov/rescue/mwr/035/mwr-035-12-0584.pdf

[2] Nakaya, U. (1954) Snow Crystals: Natural and Artificial (Harvard University Press).

 

This morning it just barely dipped below freezing, the first time in several days. Off I went to the usual black cars. And once again the frost to me looked like things I’d seen before. I decided to take a few pictures anyway, and once again I was surprised at what I saw in the zoomed images. To my eye, the site in the image below looked like small droplets that froze and then grew hoar.

But the camera revealed a little more variety. The site looks like a miniature seashore with a bunch of white shells of various shapes. The clumping of crystals is a little puzzling, as the close-ups below show no obvious resemblance to a frozen droplet. In some clumps, the hexagonal sides of the crystals are clear, in others, the crystals appear to be tilted up on end, such that their hexagonal sides are not shown in profile. In one case on the left, the crystals are rounded in outline, and some are not clumps at all, just a single crystal. (As with all images in this blog, a click will enlarge them.)

With my camera battery running low, I moved on to the tubs and rice fields. The latter had some ground-freezing with a strange lattice/mesh-like design. I see this often, but this morning’s ground had particularly striking designs. One of them looks like crabs.

Some of the lines in the ice (i.e., the arms of the ‘crabs’ above) appear to consist of ice surrounding a small twig. But by breaking some, I found that some of them do not. Their needle-like shape and horizontal orientation probably arose in a thin pool of water surrounding a wet dirt clump. When liquid water freezes in a small puddle, it does so from an edge, where it sends out a thin, straight blade of ice with a depth equal to the water depth that lies below the freezing level. Subsequently, the frozen dirt clump was pushed up by the same process that makes the brown ground ice columns I discussed in my previous post. This lifting-up brings the horizontal, ice needles as well. Later, hoar starts forming on them, making them white. At least, that is what I think happens. Some of the horizontal mesh of white needles can look rather pretty.

- JN

 

The Story of Snow has been awarded a 2009 Blue Ribbon from the Bulletin of the Center for Children's Books! You can see all the details here:

http://bccb.lis.illinois.edu/blue09.html

- Mark

A light snow has been falling for the last few days. It's not been much. I look out my window at the lawn mowed last fall, and green tips of grass blades poke out of the snow. On the internet I see the lake effect snow bands playing out to the north, but they seldom wander down here.

But for a few hours tonight a light, fluffy snow fell. I managed to get a few photos, and this in one of them. As always, click on the image for a larger view.

On the morning of December 7 of 2008, I saw a small yet distinct white patch on the ground amongst the dirty brown crunchy soil-ice columns in a rice field. If I hadn’t been looking straight at it, I would have missed it. Crouching down and clearing away some of the surrounding dirt-ice columns, I found it to be a white ice column resting on a small pebble, unlike the surrounding brown columns that rested on the soil. You can see this pebble and ice at the upper left in the image below.

The white ice “cap” detached easily and cleanly from the pebble (See the above image, upper right). I could put the cap back on the pebble, and it would stay in place, fitting snugly. Looking around the field of ice, I spotted another white cap, though not as tall as the first one. On other mornings with frozen ground, I would visit the same field, sometimes finding several white caps and sometimes finding none. The longest columns were about an inch and a half (3-4 cm) and relatively rare; more common were just white glazes on pebble tops. I could see that this ice was not hoar frost – it was too thick and dense – and closely resembled the dirty ice that pushes dirt, plant debris, and pebbles up off the ground. Sometimes, the ice grew off the pebble at a sharp angle, like the top two cases in the image below. The view is from above for the pebble at the upper right, all others are side views.

I immediately started to wonder how the water built up on top of a rock. There had been neither rain nor snow on the previous night, so the water must have come from underground, as it does for the dirtier columns.

Before addressing the new observation, let's discuss how the more common dirty columns form. I understand that the water crystallizes slightly above the freezing level in the ground; above this level, the ground is below 0 degrees C (32 F), being cooled from above, and below this level, the wet ground exceeds 0 degrees. During the night, the freezing level will start at the ground surface (which is not ‘level’ in the sense of being flat and horizontal) and slowly works its way deeper underground. This latter motion of the freezing level may be the reason that dirt and rocks get trapped in the ice, but otherwise is unrelated to the upward growth of the ice. Though I am not clear on exactly how this growth happens, I know that it has been simulated in the laboratory by substituting a thin membrane filter for the soil. In those experiments, water can flow through the pores of the filter from the liquid below, but ice cannot grow through the pores from above, at least until the temperature becomes cold enough. So, for a period of time, water flows through the membrane and freezes, and as more water flows through, more ice gets pushed up. This phenomenon has nothing to do with the expansion of water upon freezing – it is purely an ice-growth and water-flow phenomenon. In the soil, the spaces between the grains act as the pores in a filter. A more recent idea is that the boundary between ice and any other material (including air, stone, and dirt grain) has a thin region in which the ice molecules can move around in a somewhat mobile state between that of ice and liquid water. I’m not clear on how this mobile water layer influences the formation of ice columns, but a lot of articles have been written on the topic, and if I spent the time, I could probably understand it.

When I first saw the thick ice on these pebbles, I first thought of the recent idea of a mobile water layer. The pebble, no matter how clean, would have a thin coating of water before any ice formed on it. After some of the water film on the pebble’s top froze, there would remain a mobile layer of water separating the ice from the stone. More water would flow along the interface from below to replace the water that had frozen, and this would push the ice up. I showed some pictures to Charles Knight at NCAR, he told me that he hadn’t seen the phenomenon himself, but in the same year another person had related a similar observation to him. He wondered if these pebbles were porous and suggested that I collect them.

This season, I had to look elsewhere for these pebbles, as the farmer covered up my previous spot with rice husks. The bottom two cases in the image above shows pebbles from two new places. On the left, ice on a large, soft, red rock in dirt, and on the right, a few rocks from a sandy creek bed. So it appears that the white ice-on-a-rock can form in a range of places and on a range of rock types. I still don’t understand how the ice gets there, but just yesterday morning, I saw ice that seemed to have come from water within a rock. As I was walking back from a morning icespotting outing, I saw something strange on the walkway.

The ice in the above rock looks like the ‘sap crystals’ that grow out of woody plant tissue – very fine white curly ice hairs. When I scraped some of them off, I saw that the ice seemed to be loosely attached to the flat rock surface.

This appears to be ice that formed from water that came through pores in the rock, as Charlie had suggested. This ice is finer and less dense than the previous white columns, so I wonder if the thick columns formed by the flow of mobile interfacial water between ice and stone, and the finer, less dense ice in the above image formed by water flowing within the rock.

- JN

The January thaw lasted extra long this year, with temperatures yesterday topping 50F here in Kalamazoo. But in the winter the warm days are usually precede a cold snap - and it's cooling off a bit now. A little snow is actually blowing in the air. Let's hope for snowfall and a chance for photos. Above is one taken three weeks ago, on January 4, 2010, the last opportunity for snow crystal photos hereabouts. - Mark

Though I appreciate seeing the old and familiar, when I venture outside on frosty mornings, I usually see at least three unexpected things. Three unexpected things before breakfast. A few days ago, the frost at first appeared more hoary than curvy, but when I peered over the top of a black SUV, I saw ice curves in the shape of an eye. Just for fun, I put an image of it next to a mirror-reversed copy, to give the following composite.

Call it the eyes of frost. Like me, you've probably seen curvy growth before, even if it didn't take the form of an eye. But let's venture into the eye of frost and notice something new: straight-segmented web-like growth.

I've never seen that before, and I never expected it. Notice that the boundaries of the straight growth are parallel with the six prismatic faces of the larger crystals that stick up. So it seems that these web-segments grew from the vapor phase, not from a liquid film. Growth in liquid water does not produce prismatic faces, at least I've neither seen nor heard of it happening; for example, notice how the perimeter of the ice star in "They came from out of the tub" compares to that on any of Mark's snow crystals: the former are smooth and rounded, the latter straight. Moreover, curvy blades form when ice grows in a liquid at a surface. So, what I think happened is this. The car had a thin coating of liquid water that froze. But freezing wasn't uniform (it almost never is); rather, thin curving blades of ice formed. When an ice blade grows through the film, it forms a surface for ice to deposit from the vapor. Probably much of this vapor initially comes from evaporating liquid next to the blade, drying out the nearby film and preventing ice from growing there. Two such blades of opposite curvature formed the outline of the eye, and the interior film evaporated. But the dry car surface (as well as the ice blades) was still cold enough for net vapor deposition. So, from the ice blades, some crystals grew inward into the eye's interior, staying next to the cold surface. It's just a guess, though. I wish I could have had a time lapse microscopy set up for things like this.

Frost on a car roof differs from that on the hood. For one thing, the roof probably gets colder due to it having a greater 'view' of the clear night sky. Down on the warmer hood of the same car, I saw these little islands of columns - like towers of a tall castle surrounded by a huge moat.

Amazing how clean and dry it is around the columns. The ice-free moat around the towers may be a bit similar to the interior of the eye. I'll guess that the original water film that condensed here was thinner than that on the roof. In isolated spots, ice nucleated on some small, unseen particle. If the surface temperature was about -4 to -8 degrees C, which is consistent with the air temperature I measured of about 1-2 degrees, and the ice was oriented to grow as a column off the surface, then a column, or column-cluster would shoot up off the surface, as observed. If the film was thin enough, growth of ice in the water film surrounding the towers would be relatively slow. The growing tower would have sucked water out of the vapor, causing the film to dry up near the towers. Ice that nucleated further away may have spread through the liquid film, but stopped at the dry moat. Once again, I can only guess. If the roof was a degree or two colder, then the hoar would no longer be columnar. Such growth agrees with what I saw. It is consistent anyway.

-JN