Mystery of Whirlpool Hollows

February 3rd, 2010
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

BEDFISH: Revising an old Idea for Classifying Surface Ice Forms

January 31st, 2010

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.

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Like Seashells at a Seashore

January 30th, 2010

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.

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Ice on the Rocks

January 27th, 2010

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.

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Eyes and Dry Moats

January 22nd, 2010

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.

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The Maltese Cross in Pond Ice

January 21st, 2010

A few days after my encounter with the pawprints and grey muck, one of the ponds did freeze over. After me and my camera spent about 30 minutes admiring this rare event, I went and ruined the complete glaze job by punching a hole in it. Although the top surface had lots of interesting curves and shallow grooves, the underside, only about 8-mm below, was flat and featureless. I suppose this is because the surface marking the melt-line (i.e., 0 degrees C) is flat. But when I put a piece of pond glaze between two polaroid sheets, with one polaroid 'crossed' to the other, I saw an odd sight.

The large black "X" that appears here is sometimes called the "Maltese cross". Elizabeth Wood, in her classic little book "Crystals and Light", calls it the "black cross". And when I looked up some cross shapes online, I thought the above figure looked more like the German "iron cross". Anyway, whatever you call it, it doesn't seem to fit the scene.

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Strange Pawprints in the Ice

January 15th, 2010

On account of the recent cold spell, I went up the hill behind us in search of a glazed-over pond. Ponds can have interesting freezing patterns and, if the ice is thick enough, a little excitement. I've never seen these ponds freeze, but then again, I've never thought it likely enough to warrant a visit in winter. The first two ponds I visited (technically, man-made reservoirs for flooding the rice fields) had ice only over a small section at one end. I had better luck with with a drained pond. This 'pond' looked more like a dirty mud pit, but I saw evidence of ice on one side. So, ignoring the warning signs (more on that later), I walked down and around to the icy end. From eye-level, and even from a crouch, this ice looked like snow.

But except for the holes that exposed the dirt underneath and the twiggy matter on top, the surface was smooth and level, like that of a glazed pond or puddle.  Indeed, a partially glazed-over pond is probably what it had been. The whiteness was due to some fuzzy growth underneath that I could see in the large gaps (see the image below).

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Snakeskin Frost and Other Curiosities

January 12th, 2010

You can see a lot on just one black car. The nearly uniform appearance of white frost shows, upon closer inspection, a variety of forms. The first black car of yesterday morning didn’t seem so striking, and I considered passing it by. But, thinking of something that happened a few weeks ago, I hung around and took some pictures. You see, a few weeks ago, I discovered in my morning photos one picture that looked like snakeskin. It was the only one like it in my entire collection, but unfortunately a bit fuzzy. So, I swore thereafter to always look at the photos as I take them, then if something really new comes along, I can take many shots and hope that at least one is in good focus. Luckily, I stumbled upon the snakeskin frost on this car, but once again, only took one shot (alas - it also looks fuzzy). I guess I'll never learn.

If you click on the picture and zoom in, you'll see that the crystals are shaped  like thin disks, laid down nearly flush against the car roof, like the scales on a snake’s skin. The bright crystals are the ones oriented to reflect the sky, whereas the dark crystals have a different orientation. These disks are tabular forms like the hexagonal plate, except without the flat, prismatic sides. But the same roof also had columnar forms:

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Frost World

January 9th, 2010

Whenever I am taking a picture on macro near the maximum close-up to an object, the aperture on my little Canon automatically sets to the maximum size, which means that the resulting  images  are in focus only at the center. Ever since I got the camera in late 2008, I've wondered how to get past this problem. In flash mode, the aperture decreases, extending the area of focus, but the image gets overexposed. Yesterday morning, I finally hit upon the (obvious) solution: set the camera to flash mode, but block the flash with aluminum foil. I had thought before that the result would be too dark, but with some white hoar in the picture, the image is bright enough. (Later I found that the image is fine, though a bit dark, even without hoar.) For a reason I don't understand, the exposure time is still relatively long with the flash on and the ISO value set low, so I probably need to better stabilize my little makeshift wooden 'tripod' and set a longer delay to allow vibrations enough time to damp out. Even so, the improvement gives me more room to explore in the miniature world of frost, which I call 'frost world'. For example, the image below, taken on the roof of a black car yesterday, shows some pyramid structures sticking up, a few valleys, straight ruts, straight streets, and parallel curving streets. The basic structure was laid down when the film of condensed water froze, which I call "film frost" (though it has no standard name) and then later hoar crystals grew up off the surface, adding a little white to the landscape. In this image, nearly all of the hoar is hexagonal.

When I took a picture of a frosty leave, the image in my viewfinder had an odd out-of-focus look (below), so I took a bunch more shots.

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They Came From Out of the Tub

January 8th, 2010

Last night it rained, but the skies cleared before morning, letting the air temperature drop to about 2 degrees C. The weather, it seemed, would be perfect for lacy white frost patterns on cars. But it didn't quite turn out that way. A little bit before sunrise, I went out on my rounds. I took a few pictures of ice on parked cars and then headed for the rice fields. Neither tub had ice, so I walked to the abandoned car on the other side of the roadway. Nothing of note there either, but in those few minutes a blade of ice had started across one tub. Soon there were several. With a stick, I went under one blade, lifted it up, and noticed the thin serrated fingers that had been growing into the water, hidden under the surface. The disturbance of the water surface caused the growing mesh of ice lines to drift, unattached to the tub walls. I don't see this in puddles. In the puddles, the ice is always anchored to the shore, presumably because the ice starts from the shore, where there are more things to nucleate from.

In the tub I saw a curvy piece and fished it out. Laid out on the rusted lid of a nearby barrel, the ice looked strange, like a fossil of some prehistoric creature.

The critter is about 3-4 inches head to tail and paper thin. As the crystal grew from liquid water, called 'melt' by material scientists, it is said to be "melt-grown". I call such crystals "puddle crystals". Anyway, soon ice was starting on the other tub. Out in the middle of an open patch of water, I saw a small hexagon. With a stick I tried lifting it out.  I  found that the central hexagon had six large branches that had been hiding just under the surface. But the crystal slipped off my stick and cut under the water's surface - never to be seen again. Needing something better than a stick, I found a piece of a plastic lid that had been left in a nearby ditch and tore off a piece. Then next time I saw a small, isolated hexagon, I lifted it out and put it on the barrel lid.  See the photo below.

Though the central hexagon that I could see on the water was less than a half inch across, the full crystal was about one and a half inches across. Curious how the water would cover up all six branches symmetrically...  Anyway, despite the large width of our 'puddle star', it too is paper thin.  And being so flat and featureless on both sides, it evenly reflects the bluish skylight. The serrated pattern on the branches make this crystal a dendrite, like the dendritic snow crystal. But a puddle crystal grows much faster than a snow crystal because the melt (i.e., liquid water) is more densely packed with water molecules than the air from which snow grows. Also, when a snow crystal branch grows, it removes water vapor from the nearby air. This depletion of vapor is most severe near the base of the branches, and thus snow-crystal branches hardly widen - they mainly grow at the tip and at the tips of the side-shoots (called side-branches). But the melt is never depleted of water molecules, so the branches of puddle crystals keep widening until they merge. Another difference between puddle crystals and snow crystals is that puddle crystals viewed broadwise have a curving outline. The snow crystal usually has a polygonal outline, that is, made from straight lines.

- JN