Category: "Snow Science"

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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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

 

The Crystalline Beard

January 5th, 2010

I have very often in a Morning, when there has been a great hoar-frost, with an indifferently magnifying Microscope, observ'd the small Stiria, or Crystalline beard, which then usually covers the face of most bodies that lie open to the cold air, and found them to be generally Hexangular prismatical bodies…

The above passage, from Robert Hooke's Micrographia (1664), describes almost exactly what I do on sufficiently cold winter mornings. Though I don’t have a microscope, I have a Canon Power Shot camera with the macro feature that allows me to zoom in on tiny objects. (The best gift I ever got, and possibly the best thing an amateur naturalist like me could ever receive.) From afar, most hoar frost looks similar – a dusting or coating of white on any surface that can cool enough for vapor to deposit. The effect on an object can be beautiful, looking like an artist has applied a white highlighter to the small protrusions and ridges on the object, as in the rock below.

The rock is about an inch and a half across. The crystals range in size, but the larger ones are about as wide as three thick hairs (300 microns) laid side-by-side. One can see in the enlargement below that many of the crystals have a hexagon shape – like a snow crystal.

A better example showing hexagonal crystals is in the frost I saw on the roof of a black car a few days ago.

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White Snow, Black Snow, Pink Snow, Blue Snow

January 3rd, 2010

Hands down, my favorite book for browsing is M. Minneart’s The Nature of Light & Color in the Open Air. This small, easy to carry (and cheap if you get the Dover edition) book has 233 short sections on things one can see 'in the open air'; for example, twinkling of stars, the color of lakes, and the rainbow. Section 93 is on black snow, in which he notes how the apparent whiteness of a falling snowflake seems to change from blackish to whitish when the flake’s background changes from the grey sky above to a darker background below (such as dark-green trees). It is a simple enough observation, but I didn’t bother looking for it until a few years ago. Indeed, the illusion of whiteness change is strong. Look for it next time you go out when it's snowing.

On a walk on Jan. 2, I took the picture below because I thought the sun poking through the cloud was interesting, and then later realized that I could easily compare the whiteness of the flakes. Minneart's next section describes a related contrast illusion that also can be tested in the picture - the feeling that the snow on the ground is brighter (or whiter) than the sky above. See if you can shake this illusion next time you go out in the snow.

The sky in the picture is not uniformly grey, as Minneart assumes in his discussion, but is a yellowish-red at and below the level of the sun. So, for the comparison of the sky brightness to the snow brightness, I chose a patch of sky of sky at the place within the green box marked 'A' in the upper left, where the sky is darker. (To see this and other boxes, you can click on the picture to enlarge it.)

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Ice of Hearts

December 28th, 2009

Back when I lived in Boulder CO, I worked with Charles Knight on developing a new way to grow ice crystals for experimental study. I knew that the problem with most methods was twofold: there were too many crystals too close together to be able to learn how each one behaved on its own, and the surface that held the crystal would influence the crystal’s growth. Charlie hit on a great way to eliminate the first problem: put some water in a small pipette (like a narrow, tapering straw) and freeze the water from the wide end. At the tip, which stuck out into a small observation chamber, ice would grow out as a single crystal. Unfortunately, the smallest pipette tip was about a millimeter in diameter, which is too large. I then tried using Dupont hollowfill fibers, which are about 20 times thinner – about as thick as fine human hair. But this was still too thick. So I started using glass capillaries, which I could heat with a small torch to draw down to sizes 100-1000 times thinner than the pipette – about as thin as small cloud droplets before they freeze into ice. We had our method. From the start we would see things that hadn’t been reported before. Some of these things we (or I) published, but most of the things still remain unpublished. One of them is the heart-shaped crystals. The photo below shows the tip of the capillary, which is about 10 times thinner than human hair, along with a thin heart-shaped crystal.

 

 

After the heart grew a little more, it developed into a hexagonal shape. But probably the most bizarre thing I saw occurred when I decided, just for kicks, to try to evaporate ice from the inside of a crystal by connecting the wide end of the capillary up to a vacuum pump.

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Colors of Ice

December 18th, 2009

The farmer's bathtub finally froze over. The surface had an interesting freezing pattern, but I've photographed similar ones many times before. So I picked up a rock, broke the surface, put a chunk between two crossed polaroid sheets and shot a picture.

The colors arise from the birefringent nature of ice, which means that light can pass through crossed polarizers if ice lies in between. But only some colors can make it through both polarizers.

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First Frost

December 15th, 2009

Here in this neighborhood of Japan, we finally had our first good frost day. By frost, I mean any ice that forms from vapor that condenses (wet or dry) onto a surface. We rarely get snow, typically just one or two short-lived, wet snowfalls over winter, but we often get frost.  Frost might be common here because the temperature just dips a little below freezing (0 degrees Celcius), winter skies are often clear, and the humidity is high. For example, in my yard, about seven feet above ground (where I have a safe place for my temperature/humidity meter), the relative humidity last night stayed between 80 and 87% and the air temperature got down to 1.6 Celcius. But the temperature on surfaces exposed to the sky got colder. For example, on some metal plates I put in our carport, the temperature reached -6.0 Celcius. Roofs of cars parked in a more exposed area probably got even colder. The pictures below show frost from two black car roofs.

These pictures show a mixture of two types of frost: windowpane, or film, frost, which is often clear and curvy, and hoar frost, which is white and straight. Film frost grows along the surface; hoar grows out of the surface. For the snow crystal fan who lives in a place with little snow, hoar frost is the next best thing. I say this because hoar grows just like snow - sometimes like a branch of a thin star and sometimes like a column. Film frost is ice that grows in a thin film of liquid water that condensed like dew on the surface. For reasons that remain mysterious, film frost usually curves. After the liquid in the film crystallizes, hoar crystals sprout off the ice, growing upward, away from the surface. In the top image, hoar has just started, but enough has grown to make the curved ice white, in good contrast with the black surface below. In the bottom image, if you look closely, you can see the stubble of hoar whiskers growing as tiny ice columns off the surface (at this magnification, it is hard to determine if the hoar consists of columns or thin-star branches).

The thing that is totally bizzare about film frost is that the ice not only curves, but each curve consists of a crystal structure that twists. Hoar and snow do not twist. To picture what I mean by twist, imagine holding a huge snow star and twisting each branch in such a way that the crystal resembles a six-blade propeller. Real snow doesn't twist, but film frost does. Because the hoar grows off the twisting ice, the hoar columns tilt differently in different regions. You can see this if you look closely at the bottom image.