Martini Hoar (raise a tiny glass?)

October 19th, 2019

The hoar-frost crystal shoots up like a thin, solid straw, then suddenly opens up into a cup-like shape. I have seen it often enough to give it a name: "martini hoar".

Martini Hoar (raise a tiny glass?)


The cup can be weirdly segmented and polyhedral, but it nevertheless widens suddenly. Here are a few more (Sorry for poor photos—someday, I hope, I'll get better about photography.)

Martini Hoar (raise a tiny glass?)




Here is a larger view of the region. Note the similar hoar coming down from the top, but without a clear view of the base.

Martini Hoar (raise a tiny glass?)


This sudden widening feature has bothered me for awhile, but I was delighted the other day to figure out a plausible reason. My delight was made even greater because the reason involved measurements I made in the lab two decades back. The measurements were to understand snow-crystal habit, but apply equally well to hoar frost because hoar grows just like snow except it is attached to the ground.



Now that I have viewed some of the older pictures I took, the actual growth phenomenon looks more complicated in terms of crystal shape, so I am not so sure my reasoning explains things so simply. Nevertheless, it should apply well to many cases, and at least is worth learning because it involves important growth principles that also apply to snow.

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Drainage Lines on Snow

October 18th, 2019

Last night I dreamed of a passage along a narrow river in a thick forest followed by a perilous descent down steep snow on a mountain.

And then this morning a friend sent me this picture.

Drainage Lines on Snow


Actually, I had asked him about the photo yesterday, so that explains my receiving it. I had asked because the lines in the picture had originally puzzled me. They appeared to be flow-lines, but this is a snowfield, not a glacier, and even if it was a glacier, the cause of the darkening wasn't obvious. But after talking about them with Steve Warren at the UW on Monday, I came to a simple conclusion. They are indeed flowlines of a sort, but it is not the ice that flowed. Rather, it is liquid water. That is, they are channels created by rainwater drainage.

Still, it doesn't detract from their beauty, does it?
And, in a weird way, it connects to my dream... as if my mind had assumed the form of a raindrop and fallen onto the snow.



--JN

Thinking Laterally in Crystal Growth…and in Science Publishing

October 10th, 2019

After a long period of work (on and off), we have an accepted paper on the corner pockets we discovered (see here). But during the writing stage, I thought about collecting earlier ideas I had developed during my correspondence with Prof. Akira Yamashita in Japan, and as a result, the paper ballooned. Originally, it was to be a short note about the pockets, but in the end, the central theme was instead the new notion of the lateral (or sideways) growth of crystal facets. Actually, one type of lateral growth had been long known, sometimes called "facet spreading", but we collected together three types of lateral growth and describe how two of them are particularly useful for explaining a wide range of observed features on snow and ice crystals.



But before describing some of our findings, recall that all science publishing costs money. Almost always the research grant pays the page charges. However, our grant ran out three years ago, and this paper is particularly expensive (estimate: $3600) because we needed many pages to argue our points and show how lateral growth can explain many growth forms. If you can contribute, we will gladly send a signed copy of the paper acknowledging your help. Or, if you have just enjoyed some of the articles on this blog and would like to help me continue, you can contribute here as well. Here is the link:

https://www.gofundme.com/f/publish-a-research-paper-on-snowcrystal-formation


The paper in its submission format is 58 pages: (as always click on the image to enlarge)

Thinking Laterally in Crystal Growth…and in Science Publishing




The published format will reduce the length a little (mainly shrinking the figures), but it will still be much longer than the standard 10-15 page paper.



One key figure to help explain some of these "lateral" concepts appear in Fig. 2, reproduced below. This figure shows a just-frozen droplet, which is often called a "droxtal". Crystal facets have appeared for eight faces, shown as the shaded flat regions in the top sketches.

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Dark lines in melting snowpiles

March 30th, 2019


Snow cleared from a city street slowly vanishes, leaving lines of "dirt".

Dark lines in melting snowpiles


(Click on images to view more closely.) Where the snow has a distinct edge, you can see the dark lines are ridges, some of which can be quite sharp:

Dark lines in melting snowpiles


Why is this?

Cryoconite ridging

This all happens because the snow is vanishing but the dirt is not. The vanishing is actually called "ablation", meaning some combination of melting, sublimating, and evaporating. It is mostly melting in the above case, but it is possible that evaporation helps to form the lines. About the dirt, it is not clear exactly what it is. The term "cryoconite" is used when similar dark dirt falls and clusters on glaciers and ice sheets, so to be specific, we might call it "cryoconite ridging".

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Poster on corner pockets in snow crystals

February 18th, 2019

Last July, I attended the AMS (American Meteorological Society) 15th Conference on Cloud Physics, which was combined with a similar conference on atmospheric radiation. They have these about every four years, but I have missed all those going back to 1995. So, I had a lot to catch up on. Actually, by far the most enjoyable part of this conference was meeting old colleagues and making new acquaintances, including many who I had known only through their papers and email correspondence. As to why I waited so long, I suppose the main reasons were time and cost. Even without travel and lodging expenses, conferences are expensive. This one was $600, and the fee for the abstract I submitted was an additional $95. But in this case, the conference was only a 3-hr drive away (Vancouver, BC) and my co-worker very nicely picked up the conference fee and hotel tab. Also, this conference had a special session dedicated to the work of my former advisor, Prof. Marcia Baker. In that session, I presented the following poster.

Poster on corner pockets in snow crystals


I've described a little about corner pockets in a previous post, and showed a few of these panels before, but will briefly go over them here.

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Puddle gets its grooves (upon freezing), part I

February 16th, 2019

These grooves appear on the surface of frozen puddles.

Puddle gets its grooves (upon freezing), part I


Most grooves are straight lines, and most of these also appear to have relatively symmetric sides, such as those marked #s 1, 2, & 4 in the above image. But some, such as #3, have sides that are much wider. Grooves can intersect each other at a "T" intersection, such as #1 and #3, or they can cross, as a 4-way intersection as in #2. Some, such as #4 start and end on nothing apparent and intersect no other grooves.

Sometimes the sides show small steps or sub-grooves, particularly on the wider sides, such as on #3. On grooves that curve, such as in #1 and #2 in the image below, one side can have a fern or foliage type of texture.

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Strange dry shadows

February 14th, 2019

Sunshine on pavement seems straightforward enough. It warms the surface, driving evaporation (or sublimation), and dries things out. Conversely, we know that moisture tends to collect on surfaces that are either colder or closer to a water source. And the moisture tends to persist more if the region is more enclosed. Agreed? Now, does anything look strange about these shadows?

Strange dry shadows


The shadows of these snow clumps appear to be dry pavement, the regions in the sun moist. As the sun moved across the sky, the shadows moved, and once dry pavement that was formerly in the shadow became moist as it came into the sun. The image below was taken three hours later.

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A rare heliac arc? (plus six others)

February 5th, 2019

The day starts sunny and bright, but later you note a slight muting of the surrounding landscape. It is still bright enough, but you feel less heat bearing down. Apparently, a thin veil of high clouds has slowly and silently appeared above.

When this happens, please take a few seconds to look up and see what these clouds are up to. Do they have their crystals lined up for one or more arcs, or are they oriented randomly, giving a halo with perhaps a sun dog or two? To the trained eye, the chances are high that such a veil will give rewards, even when no crystals are present. But the crystals give so much more, particularly when they line up in various ways.

Wandering, somewhat exhausted, on the hillside above Index, WA last Monday, I got this very sense of a muted landscape. So, I found a break in the trees, looked up, and instantly felt recharged. A colorful circumzenithal arc had appeared, which by itself is always a treat. But I was also awed by several other arcs and a very diffuse 22-degree halo. Excitedly, I took photos and scrambled around trees and rock for a more complete view.


A rare heliac arc? (plus six others)


(As with all images here, click on the image to see a larger view.)

Due to the complex way the eye and brain discerns light and patterns versus the much simpler way of a camera, the patterns are much more distinct when viewed direct by eye. But the above composition has my attempt to compensate the original photo at left, with some contrast enhancement in the middle version, and markings on the right version.

The band of color at the intersection of CZA and SLA is from the oriented prism crystals of the circumzenithal arc (CZA). Their formation is relatively common I think, though a group of observers in Germany apparently finds their occurrence there to be only about 13 times per year (https://www.atoptics.co.uk/halo/whyinfr.htm). What is most remarkable to me is not their colors, but the fact that formation of distinct colors requires that the crystals stay extremely level as they fall (deviations of only a few degrees would wash out the colors). My sketch below shows roughly what is going on here:


A rare heliac arc? (plus six others)


(Hold on for a moment, and I'll get to the rare heliac arc below.)

Full story »

Bad Snow, Part II, and Some History

December 11th, 2017

The recent post about bad snow (see new emblem at bottom) reminded me of an old image that often pops up in historical descriptions about snow. The date was 1555 and Olaus Magnus, an archbishop in Sweden, just published "Description of the Northern Peoples", a long multi-chapter, multi-volume text with the interesting chapter for us being titled "The Shapes of Snow" with the following illustration.

Bad Snow, Part II, and Some History



I am not sure what the sections on the left half represent, but the right half shows a fanciful variety of "snow shapes", including a bell, a hand, an arrow, a crescent moon, and one at least in the form of a six-pointed star. Perhaps the author gave incomplete instructions to his illustrator.


At any rate, scholars have suggested that this illustration is actually the first to show the form of snow. Though the Chinese had many centuries before correctly described snow's "six-fold" nature, no illustration has yet been unearthed before Olaus Magnus's above. Luckily, it wasn't so many centuries later that we got much more accurate illustrations from the likes of Descartes and Hooke.


Finally, as promised, the proposed "No bad snow" emblem.

Bad Snow, Part II, and Some History



--JN

Film frost grains and radiative cooling of the ground

December 10th, 2017

December 8th brought the first frost to the Seattle area. This doesn't mean that this is the first time this season that the ground reached 0 degrees C or lower. True, we had gotten snow in late November, though this by itself doesn't mean the ground reached 0 C because snow deposition differs from frost formation. The snow was below zero when it formed in the air, but for the rest of its existence, it could have been melting and the ground itself likely sped up the process by remaining slightly above 0 C. But in contrast with the snow, for frost to form, the ground itself must cool below 0 C.



When I used to put out some metal plates with recording thermocouples, I didn't see visible frost until about -5 C, but that was based on just a handful of measurements. Anyway, what this means is that we may have had a few mornings with some patches of ground (including anything connected to the ground) dipping slightly below zero but with no obvious frost appearing. Also keep in mind that "ground temperatures" reported at weather stations are 1.5-2.0 meters above the ground, and thus may be 5-10 degrees C warmer at night than some ground patches. Why? At night, the ground cools by radiating, and if the atmosphere is not radiating much down, then considerable cooling happens. This is why clear nights are the ones with the most frost or dew.



Anyway, here is one shot of some of this "first frost" on my car window.

Film frost grains and radiative cooling of the ground


The image shows patches of different texture. These regions differ in texture because they are tiny hoar-frost (i.e., vapor-grown) crystals of differing size or orientation. That is, they stick up differently in different patches. They stick up differently because they sprouted off of a thin layer of film-frost that had a different crystal orientation. So, the patchy look comes from the different grains in the film-frost. See some of my previous posts with diagrams about this phenomena (category: "film frost"). Here is one with particularly helpful diagrams.

http://www.storyofsnow.com/blog1.php/choppy-waves

-- JN