Capturing Falling Snow in a Cold Fluid

February 15th, 2021

Snow is usually imaged in air, the single crystals laying flat on some substrate such as glass. The method is relatively simple, but one must work fast to image the crystal before it appreciably sublimates. Sublimation first rounds out the sharp edges and then causes the crystal to shrink. Generally, this sublimation happens because the photographer is radiating too much heat to the crystal. Conversely, particularly in very cold conditions, the photographer’s breath may deposit fog near the crystal, causing the crystal to grow.

Such issues vanish if one instead captures the snow in a cold fluid before taking the image. To work, this fluid should not dissolve the ice, be less dense than ice, be fluid enough to completely spread over the crystal surface, and be transparent. Other than preserving the crystal, the method has several other advantages. For example, in his laboratory experiments in Hokkaido, Japan, Tsuneya Takahashi lets the crystal fall into a cold suspension of two transparent, cold silicone oils. He sets it up so one fluid is denser than ice, one is lighter than ice, so the crystal falls through the light oil and rests on the (transparent) interface between the two fluids.

This method sounds complicated, so why use it? One, as the oils are immiscible with water, they block water molecules from arriving or leaving the crystal surfaces, so the ice crystal shape is preserved precisely for as long as the fluid is below 32 F (0 C). Two, after imaging the crystal, the fluid is warmed above melting such that the crystal melts into a spherical drop from which he can easily measure the volume and thus infer the mass of the original crystal. A third advantage, more difficult to exploit yet sometimes used, is that he can get top and side views of the same crystal. Other researchers in Japan have also used silicone oils to capture ice crystals in the lab, as well as naturally falling crystals, mainly for the first and third reason. They use just one oil type, a lighter oil. Charles Knight at NCAR in Boulder, Colorado had a fourth reason for using a cold fluid: better imaging. That is, one can image greater depth detail because light scattering off the surfaces is greatly reduced, particularly if the fluid is very clear and has an index of refraction close to that of ice. By reducing the scattering, one can see through surfaces to underlying surfaces. He would use gasoline or hexane fluid.

Capturing Falling Snow in a Cold Fluid


I don’t have a photomicrography setup to take detailed images of snow, and we rarely get snowfall with nice single crystals anyway, but I wondered how well the method might capture falling snowflakes. That is, could I at least see their rough shapes as they fell through the fluid?

Here, we typically get about one light snowfall per winter (2-3"). A relatively large snowfall happened this past weekend, depositing about seven inches. At first, the particles were small, probably highly rimed single crystals or small aggregates. Later, larger snow particles fell, and these particles were clearly snowflakes (i.e., aggregates). In preparation, the previous night I set out two covered wide-mount jars, one with water, the other with Coleman camping fuel (white gas), which has similar properties to gasoline. In the morning, the one with water had frozen, so I knew the other was also below 32 F.

Capturing Falling Snow in a Cold Fluid


Outdoors, I set the jar on top of my car, put a wooden chopstick in the jar both to focus on (my camera only has autofocus) and to provide a size reference, set a small LED light panel to the side for brighter illumination, and then opened the lid. The flakes fell into the fluid, and fell down to the bottom of the jar. They fell through the fluid slower than they fell through the air, but it was still too fast for me to see how well they were focused. Turned out that they were not very sharply focused, yet one can still see their general shape and fall orientation (below). In general, the flatter the flake, the more it tends to orient broadside to the fall direction.

Capturing Falling Snow in a Cold Fluid



Obvious improvements would be a better jar, such as one with a flat, smooth glass front, a better camera, and a thicker fluid to slow down the rate of fall.



Such improvements will have to wait at least until next winter.


--JN

A Stroll on a Mildly Frosty Morning

December 23rd, 2020

So far this winter, we've had few frost days in the Redmond, WA area. This morning was typical of the half-dozen or so: a very light dusting of hoar crystals on the roofs and grass. One cannot expect much, and yet I am rarely disappointed. Indeed, it is not until I get close to some icy thing do I notice anything interesting. Sometimes, I still don't notice until I've clicked a few closeups and then viewed on a large computer screen. Here's what I found on this morning's stroll:

A Stroll on a Mildly Frosty Morning



The frozen puddle showed some curvy meniscus lines and some straight ice blades, which I've discussed before. The film frost is more mysterious still, but the main curvy pattern is due to the freezing of melt (not vapor deposition, though some of this does occur). The hoar frost shows some scroll crystals, which I recently addressed in an article (no mechanism had previously been argued, but we propose an explanation). See the crystals on the leaf at upper right for the best examples of scrolls, though the details are yet a bit too small. The needle ice is the phenomenon responsible for the crunchy dirt and pushes ice up from the bottom. The ground is warmer below the surface, and here the liquid migrates to the ice front, pushing it all skyward.


Anyway, that's it for this post. No detailed explanations of anything. If you would like to see such explanations, click on the appropriate category in the archives at the right side.


And here's hoping you have a nice, frosty Christmas, wherever you are-


--JN

Hoar Frost on Plastic

February 17th, 2020

The pattern of hoar frost depends on not just the current and prior conditions, but also on the surface. If the surface is less attractive to water, that is, more hydrophobic, then the hoar crystals tend to be more further spaced apart.


Hoar Frost on Plastic


Or even more widely spaced apart. (Click on an image to enlarge it.)

Hoar Frost on Plastic


These two images show hoar frost on plastic surfaces (garbage bin lids). If you look closely at the above picture, you will see that the hoar columns are growing off little mounds. These mounds are frozen droplets. You'll also notice that the directions of the ice columns are not random, but instead a given ice column tends to be pointing in nearly the same direction of its neighbors.

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Raindrop Hillocks and Ground Ice

February 16th, 2020

Ever see these small centimeter-scale hillocks in dirt or sand, usually topped by a small pebble or twig? To me, they look like a miniature mountain landscape. A brief reflection on their appearance suggests erosion by raindrops: The drops fall down on and near the larger grain (e.g., pebble), pushing the smaller grains down, leaving the larger ones to stick up above. In this way, a scene of tiny hillocks emerge. I call them "raindrop hillocks".


Raindrop Hillocks and Ground Ice

Click on any image to enlarge it.


Soft, easily compactable soil seems necessary to their formation. Just toss some loose dirt into a pile, then come back after a heavy rain and you are likely to see something similar. But further reflection may present some difficulties. For example, some hillocks occur where the soil should not be loose. The above image presents one such case: here the soil was on a well used trail where the soil had long been compacted. Clearly such soil could not be so easily carved by tiny raindrops. The case pictured below is on a level sandbar after a river shifted course. Where did the sand go that was up near the peaks? It seems that the sand must have originally been very loose. How could this be?
Raindrop Hillocks and Ground Ice

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The Curious World of Ice and Snow: Part 3 of 3

February 8th, 2020

These 14 slides are the final (third) section of my Science Cafe talk. (Plus two slides added as an introduction.) As in the previous section (previous post), this section mostly has ice forms that come from the melt, but the ice shapes here are a little "hairier". And at the end we return to forms influenced by the vapor. As with all these forms, I doubt any could have been predicted before their discovery. Nature is complicated, so Nature surprises us.


The Curious World of Ice and Snow: Part 3 of 3


Click on any image to enlarge it.

As before, the green font below shows the content of this section. The first five cases involve melt flow along surfaces and in narrow pores. Remember that melt is another name for liquid. (The term melt is more accurate though, as it implies pure water, whereas liquid could be water mixed with any solute. For example, salt water is a liquid, but it is not melt.) As mentioned in part 2, their formation from the melt means that they tend to grow relatively fast and large.

The Curious World of Ice and Snow: Part 3 of 3



First up is perhaps the most common. Do you know what is going on when the ground becomes crunchy?


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The Curious World of Ice and Snow: Part 2 of 3

February 8th, 2020

As I mention in Part 1, these are slides I gave for a Science Cafe discussion session in 2012. This section focuses on ice that forms directly from the melt, that is, the liquid phase. Contrast these cases with those in part 1 in which the ice grew from the vapor. Some of these cases might seem a little familiar, but many ought to seem downright bizarre.


The Curious World of Ice and Snow: Part 2 of 3


As always, click on an image to enlarge it.


The contents here are emphasized in green font below. The underlying difference between ice growth from vapor and from melt is that the melt is much denser. The higher density means that vastly more water molecules can impinge on the ice surface in a given time and area. This higher impingement usually means faster growth and larger crystals, but the way that the melt reaches the crystal influences the form, and the result is not always so obvious. In fact, I would even say that it is never obvious.

The Curious World of Ice and Snow: Part 2 of 3



OK then, let's get right back into it.


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The Curious World of Ice and Snow: Part 1 of 3

February 4th, 2020

In 2012, I gave a "science cafe" talk with a local series sponsored by the Pacific Science Center, KCTS public television, and Science on Tap. The title was "The curious world of ice and snow". The location was a bar in Kirkland, but open to all ages. When I showed up with my family, they tried to seat us in the backup room, the regular room having filled up, but I said "Oh, well I'm the speaker" and they kindly created a space for my family in the regular room. I was indeed surprised at the crowd. People are apparently more interested in ice than I thought. (Hmm, but where are they when I post here?)


The Curious World of Ice and Snow: Part 1 of 3


Click on any image to see an enlargement.

The basic structure of each talk was to give a lecture of about 30 minutes and then allow up to an hour (I think) for the Q&A. In my excitement, I had created 41 slides, in retrospect too many for the allotted time.

Given all the time spent preparing the slides, I hope that by posting them here that even more folks can enjoy the images and discussions. But, instead of unloading all of them on you at once, I will break the discussion into three sections. By adding the following table of contents, each section will have 14 new slides and the total will be 42, which according to Douglas Adams* is a really special number.

The Curious World of Ice and Snow: Part 1 of 3

The contents of this section is part "1", written in green font.

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Some "Inexplicable" Snow-crystal Features: Applications of Lateral Growth

January 29th, 2020

Last October, I gave a talk at the University of Washington about our recent experiments and ideas about snow-crystal growth. My pitch was general and short, as few folks work in this area and I'd hate to bore them with a long lecture. So, I was delighted to see quite a few graduate students in the audience, some of them asking good questions.


Instead of giving the narrated presentation here as a video, I will give the slides (23) with brief explanations similar to what was spoken at the talk. Narration below each slide. Skip to the ones that look interesting, and click on them to enlarge.

Some "Inexplicable" Snow-crystal Features: Applications of Lateral Growth

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The Growing Icicle's Hollow Tip

January 17th, 2020

If you inspect the tip of a growing icicle, you might be surprised to find it hollow. Skeptical? Well, if you think the conditions suitable for icicles outside, put a toothpick in your pocket and go outside. Then when you see a likely candidate, poke its tip with the toothpick.


The Growing Icicle's Hollow Tip


Click on the image to enlarge.

Not all icicles are growing, of course. If the supply of flowing water has dried up, the tip may be solid, or if the air has gotten warm, then the tip may be rounded and wet. The pic below shows a longer, but solid-tipped, icicle next to the growing one. Its melt-water source is too low for the tip to grow.
The Growing Icicle's Hollow Tip


Transitional situations occur as well, such as the icicle that just started to melt and still has a hollow tip, or the completely solid icicle that just starts growing again and has yet to develop a discernable hollow. The drip may also be intermittent. You might not find that growing icicle right away, but keep that toothpick handy.


As to why a growing tip is hollow, I won't attempt to prove it here, just try to make it seem plausible. Spend a few moments considering the following two simpler situations that show the basic ideas. You can even apply these ideas to other forms involving freezing water...

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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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