Category: "Snow Science"

The new ice-crystal-growth apparatus

July 1st, 2014

After a few years in the making, our new device for growing single ice crystals in a well-controlled laboratory environment is nearly ready. We are just adding a few small accessory pieces to allow us to start testing. I was to describe the apparatus at the cloud-physics conference this month in Boston, and made a poster to present, but decided to opt out. But, having spent a few days making the poster, I present it below.

As with all images here, click on image to see large-scale view.


And below is the same, but in blog format.

Full story »

Halo in the sky? Uh, I don't see no halo...

April 20th, 2014

After a few days of fine bright spring weather, the barometer falls and a south wind begins to blow. High clouds, fragile and feathery, rise out of the west, the sky gradually becomes milky white, made opalescent by veils of cirro-stratus. The sun seems to shine through ground glass, its outline no longer sharp, but merging into its surroundings. There is a peculiar, uncertain light over the landscape; I 'feel' that there must be a halo round the sun!
And as a rule, I am right.


The quote, from Minneart* describes a common ice-related atmospheric apparition. It appears in skies all over the world far more often than the rainbow, yet few notice it. As a graduate student, I read about halos and often looked for one, but didn't notice it myself until someone else pointed it out. As a post-doc in Boulder, I was out walking with Charles Knight, and I mentioned my lack of success. He glanced up near the sun, pointed, and said “why there's one right now”.

What I had missed in my readings had been the fact that most halos are rather indistinct and often incomplete circles. Indeed, now when I point out the most common one (the 22-degree halo) to someone nearby, they often don't see it. But occasionally, it is sharp enough (and colored) to the extent that anyone will see it if they bother to look up and glance toward the sun. And often it occurs with other ice-crystal apparitions that are even more obvious.

Last fall, while perched high on a rock face, belaying my partner up**, I saw such a vivid display.


The bright spot is called a “sun dog”, “mock sun”, or “parhelia”. They, one on either side of the sun, usually appear together with the 22 degree halo. Indeed, the sun dogs very nearly mark the spots where that halo intersects another arc called the parahelic circle. Their cause: horizontally oriented, tabular ice crystals.

Full story »

The fun of shooting down your own dang theories

April 10th, 2014

Thomas H. Huxley once wrote the famous line:

The great tragedy of Science: the slaying of a beautiful hypothesis by an ugly fact.


Great man and a catchy phrase, but perhaps he was being a bit overdramatic. To me, the slaying of a “hypothesis” (i.e., pet theory or just idea, really) is itself a beautiful thing. It means that one can do a lot of damage with just a simple observation. I like it, even when I'm shooting down my own damn theory. Here's an example:

Some time ago in my experiments, I saw the following ice growth sequence:


What you see there is an extremely small, thin ice crystal growing from the tip of a glass capillary into air. (Size-wise, the glass capillary is about 5 micro-meters in diameter, or about 1/10th the thickness of the hair on your head.) I saw it happen several times. As the crystal grew, it developed the prism facets that generally define the hexagonal crystal shape. Other people had seen such rounded growth before, generally within a few degrees of zero (C), though in all cases, the crystal had been extremely thin. You can also see this thin, rounded (non-facetted) form in some hoar-frost formations.

The mystery here is why the disk grows without the prism facets for awhile. I never saw this with thicker crystals, so I formed a little theory. The theory involves the source of the water molecules to the curved region of crystal: some come from the vapor in the air and some wander over from the flat, non-growing crystal faces on front and back. In the 1960s, people had tried to measure this “wandering distance”, but never determined a consistent value. My theory predicted that once the crystal thickness exceeded about twice this distance, the curved edge would transition to flat, giving rise to the hexagon.

But then I looked closely at this case:


That sequence shows the side and front view. The crystal doesn't discernibly thicken when the flat prism facets appear. Zing! That theory shot down.

On to the next pet theory. So maybe the key factor is the diameter of the disk: One might argue that the curvature of the crystal surface must be below a certain value, which means a large enough diameter is needed, for the flat facets to appear. Or, the size of the resulting prism faces must be larger than that needed to have several surface steps (which help ensure flatness).

But then I look at this case:


In that case, I see both smaller and larger prism faces forming at the same time (same thing can be seen in the previous image). So, I guess the curvature or size of the resulting face is not the main factor. Zing!

Some researchers had observed slight bending of the prism facet above -2.0 C in equilibrium. They postulated a “roughening temperature” at -2.0 C. This might explain the rounded disc edge, but wait! 1) This disc edge becomes facet as it grows, so it is not merely temperature, and 2) these crystals are below -2.0 C.
Zing again!

Well, there are always “impurities” to blame! Crystal growers are fond of blaming some trace, active chemical, or “impurity” for inexplicable results, so we could theorize that the above show the effect of surface impurities. As the crystal grows, the area over which the impurities distribute increases, thus diluting their concentration, and thus reducing their effect. Yes! This could explain these results.

But, wait, what about this:


The above shows two sequences (same capillary) in which some prism facets have formed, but some remain round. In the right-side case, one corner even rounds as it grows. Hmm, not a likely result of impurities. Zing!

So, I am down to one last theory. I do not yet have the data to shoot it down. And I'm not telling you, or I'd ruin the fun. We just need more data.

All in all, I think Huxley needs a little tweaking to serve my view:

The great beauty of science: the slaying of a pet idea by a simple observation.


-JN

The cup and the butterfly

February 25th, 2014

In early January, while visiting a cold, dry region, I saw this frost on a wooden fencepost.


The pattern resembled a cluster of butterflies. In the shade, these "butterflies" were blue, reflecting the blue sky. In the sun, they were bright white:


These are a type of hoar-frost, and though hoar-frost grows by the same processes as snow crystals, they can take on an even greater variety of forms. That they have more forms is a consequence of the fact that they can experience much greater levels of humidity, that is humidity relative to their temperature. This greater degree of humidity produces faster growth. Frost forms can also be more unusual because the proximity of the crystals alter the vapor gradients.

Full story »

Bending of branch and pond

February 11th, 2014


                Bending bending bending
                The fir branches are bending
                They are waiting for more snow




The famous Japanese poet Basho wrote a poem like the above, except it was about bamboo bending under the snow load.

I was reminded of Basho's poem yesterday, as we just had our first snowfall of the season. Here in Redmond, about 2 inches, just enough to bend some of the fir branches.

And not only the branches were bending.

I took the opportunity to check out the pond. It had an ice covering before the snow, but with the snow and warming trend, the ice had thinned, and gotten pressed down under the snow load. The glaze over the pond was bending. Where a hole appeared in the ice, water got pushed up and out, flowing in dark channels over the icy glaze, forming a spider-like or branch-like pattern in the now slushy snow. I climbed up a tree on the shore and took a few shots.

Full story »

One hundred twenty one forms of falling ice: the new snow classification system

January 22nd, 2014


When I last wrote about the classification of falling snow and ice (02/14/2010), I discussed the 1966 Magono-Lee system. At the time, this system was the most recent one, and as such, the one generally used in meteorology. And to those who wondered how many types of icy precipitation exist, Magono and Lee would tell you 80 types.

Although 80 sounds like a lot, Magono and Lee did not include at least one very common type as well as a few other interesting types. This major omission was snow-crystal aggregates, generally known as “snowflakes” by meteorologists. These are individual snow crystals that have stuck together during their fall and arrive as large open clusters of crystals, often with tens–to–hundreds of crystals in a big, round blob. (To meteorologists, a snow crystal is not a snowflake; rather, a snowflake consists of many snow crystals. It is like the difference between a trundled rock and a landslide.) In fact, the snowflake is probably the most common type of snow precipitation in most areas that receive snow. Several other omissions of the Magono-Lee that I mentioned in my 2010 posting included spearheads, seagulls, and the both the 18- and 24-branched forms.

If you were waiting for someone to fix the classification, well wait no more: we now have a new classification scheme that includes all of these and more. The new system has not one, but three types of snow-crystal aggregate (snowflakes), which are given the symbol “A” (for aggregate). There is also four types of spearhead crystal (symbol “CP8”), five types of seagull crystal (symbol “CP9”), and both the 18- and 24-branched crystals (P5e and P5f). In all, the new system has added 41 new types. In one big graphic, here is the new system:



The architects of this new classification scheme published the above table in summer 2013*. There were four authors (K. Kikuchi, T. Kameda, K. Higuchi, and A. Yamashita), making it harder to name the system after the originators, but hopefully their names will become known, just as Magono and Lee’s have.

I will discuss various aspects of this new system in subsequent postings. But for now, I point out that the number of basic types has increased over the years, from 10, to 40, to 80, and now 121. When will it ever end?

Of course, it may just keep increasing. In fact, if we plot the numbers vs the years, and look at the pattern, we might conclude “never”.



Full story »

Rime falling from branches

January 24th, 2013

While riding my bike home the other day, I saw what appeared to be a patch of light snow.


It was the only such patch around. Looking closer, I could see that it consisted not of snow, but of chunks of partly melted rime deposits. (Note how the pieces are long and narrow, like little icicles.)


Just above were the branches of a huge Douglas Fir tree. Perhaps there had been an over-active squirrel or two up there. Other trees still had their rime. Hard to see why only this tree would have its rime fallen off.

- JN

Crystal-to-crystal “communication” through vapor and heat

January 5th, 2013


Two mornings ago, I saw this on the windshield of a parked car.


The bulls-eye pattern wasn’t centered on any particular feature on the windshield, and there were similar, though less developed, patterns nearby. See them on the photo below.


The dark parts are largely frost-free regions, and thus are regions that dried out during the crystallization event. (It was still dark when I took the shots.) Later, under a brighter sky, I saw a different pattern on the hood of another car, a pattern that I figure has a similar cause. In the hood case, shown below, the dry regions are bright due to reflection of the sky.


Now, about those concentric rings …

I puzzled over a larger such pattern in the Feb. 5, 2010 posting “Ripples”. The causative process that I proposed back then seems consistent with this newer observation, but I will clarify it here.

Before the first frost formed, the windshield, though seemingly dry, nevertheless had a thin layer of liquid water. This thin film had cooled to some temperature below freezing. (See the sketch “How hoar frost forms” in the Jan. 11, 2012 posting.) As the windshield and water film continued to cool, freezing was inevitable; the only issue was where. The first such spot to freeze must have had some feature, however minute, that was advantageous to freezing. It may have been a nucleant particle (e.g., a mineral dust grain, a type of bacteria, or even a tiny fleck of ice that wafted in from somewhere else), a slightly cooler spot, or a slightly thicker water film (e.g., from a scratch or indentation). But for whatever reason, the ice formed there first.

Now suppose the ice spreads outward from the nucleation spot in all directions with about the same rate. I suspect this happens when the film is extremely thin, as it would have been under the relatively dry conditions of this day. So, the frozen film grows outward, roughly as a disc. See the sketch below; the black dot in the center is the first place that froze.


Full story »

Trip of the Ice Man

November 9th, 2012


The "Ice Man" -- that's how a newspaper header referred to me after I gave a recent conference lecture:


A link to the article is here:
http://www.sctimes.com/article/20121023/NEWS01/310230011/Featured-speaker-details-snow-crystals-SCSU-storm-conference

It was a very enjoyable visit to the 7th annual Northern Plains Winter Storms Conference on the campus of St. Cloud State University in St. Cloud, Minn. We had various talks including one about forecasting a storm, the statistics of the snow-to-liquid equivalent ratio (e.g., in some areas about 13" of snow will melt to 1" of water, but regions and storms vary considerably, some being over 70 to 1),and one talk about how a late-season snowstorm might end a locust plague (unlikely, according to the speaker).

My talk described how a simple principle allows us to understand how a wide variety of snow crystal forms originate.



Here's the narrated talk. To view, click the image, then enlarge to full screen:


Part 1 (10 min):
-> Why study snow crystal shapes?
-> Some history about snow science.
-> The habit map.
-> Questions that will be answered in the rest of the talk.



Part 2 (13.5 min):
-> Basics of snow crystal science.
-> Why dendrites grow so thin.
-> Why needles and columns sometimes form.



Part 3 (6 min):
-> How the crystals get their branches.
-> Why they are six-fold symmetric.



Part 4 (14.5 min):
-> How the crystals get sidebranches.
-> Common errors we make when drawing snow.
-> Why they have so much variety.
-> Mysteries about snow.
-> What are the "messages in water".



It is a scientific talk, so it involves some diagrams and technical terms. But this one is pretty easy. Perhaps the only technical terms are "vapor deposition" and "supersaturation". Vapor deposition happens when water molecules in the air (i.e. water vapor) crystallize onto something, like a snow crystal or hoarfrost. The vapor must be "super" saturated for this to happen. Greater supersaturation means greater vapor density and thus faster growth. The above link goes to the first segment, and from there you can click on the subsequent segments.

On the flight home, I saw a subsun on the clouds below. A subsun is a reflection of the sun from tiny, flat, hovering plate-like (tabular) ice crystals. They are essentially hovering like microscopic flying saucers.



In the photo above, you can see the sun's reflection off the wing on top. The smaller reflection on the clouds below is the subsun. I used to think the subsun was rare, but apparently I simply wasn't looking. This subsun was there in various forms for at least 2/3rds of the flight. I've seen them on most previous flights. More on subsuns in the next post.

- Jon


Here's the abstract to the talk:
snow crystal seminar abstract.pdf

And here are the four parts of the talk in pdf form (from the PowerPoint slides):
snow order and mystery - 1of 4.pdf
snow order and mystery - 2 of 4.pdf
snow order and mystery - 3 of 4.pdf
snow order and mystery - 4 of 4.pdf

Finally got it!

February 8th, 2012

For a few years now, myself and another ice-researcher have been trying to get a research grant from the National Science Foundation to study snow. Well, we just recently heard that the funding came through and I’ll be starting on the project in March. Yeah!

You might wonder what it is that we don’t already know about snow. Well, the better question is, what do we really know? The answer is “not much”. There’s been a lot of experiments in the lab, spanning nearly 100 years now, with many interesting results, but nothing really clear has emerged. Here’s one of the basic problems: In a typical cloud consisting of supercooled droplets and snow crystals, we can calculate the crystal’s rate of growth fairly accurately if we know the crystal shape. But it is extremely difficult to predict the crystal’s shape, and the rate of growth can change a lot with the shape. That’s the thing about snow crystals - they come in a bewildering variety of shapes, and we still have no clue as to why. And if the crystal isn’t surrounded by supercooled droplets, the situation is even worse.

We argued in our proposal that the main reason we don’t have a clear picture about the development of snow crystal shape (and even growth rate) is because of problems with the experimental methods. Instrumental influences easily creep into the experiment and alter the crystal shape. We suggested a new method based on one I helped developed in 1994-1996. In that method, we grew single ice crystals from the tip of an ultrafine glass capillary (about 10x thinner than the typical hair on a person’s head). Some of the images below show what I mean about shape variety.


The crystals are all very small and compact because we were interested in growth with very low humidity. In the series (a1) - (a7), you see the crystal growing as I rotated the capillary to see all of the crystal faces. Sometimes I let a crystal grow for a week or more.


You can see that sometimes a crystal is 5-sided. But the angles between the prismatic faces are always multiples of 60 degrees. I don't think I'll ever see a 5-sided snow crystal that looks exactly like a pentagon.


The large crystal at the bottom left & middle was starting to melt. The melting formed cusps on the edges of the face. The one at the top right is called a bullet rosette. These crystals are very common in cirrus clouds and other very low-temperature clouds. The sequence at the upper left shows something we can do with a capillary: we can suck the crystal out from the inside. In that case, I put a vacuum on the other end of the capillary and a void started to form. Then, inexplicably, a crystal started growing inside the void! You never know what will happen in these experiments.

By this time next year, I hope to be able to show new crystals we grew. And sometime soon after that, have some reliable results.

-- Jon