Category: "Ice Science"

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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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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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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