Category: "Ice Science"
To understand snow formation, one must know a little about clouds.
Q: What is in a cloud?
A: Air, dust, vapor, droplets, and often, ice.
Q: How much air? How much liquid water? How much ice?
A: The answers will probably surprise you. See my short 20-min presentation below. I gave this recently to the Bellingham, WA Snow School. (23 slides, but due to file-upload-size restrictions, I had to put them into three parts below, 10 slides, 6 slides, 7 slides.)
Snow, rain, and weather affect everybody, yet how many of us learned in school even the most basic facts about precipitation in school?
Q: Who first realized how ice grew in a cloud?
As described in my presentation, he realized this by observing frost on the ground.
Q: Who first realized how Alfred's theory was intimately connected with rainfall?
Tor discovered this by observing fog in a mountain forest, and like Alfred, applied some of his physics knowledge.
In my presentation, I discussed Alfred Wegener, the roles of the different cloud components, and briefly how the ice, once formed, takes on its strange shapes:
First 10 slides (with blue text added to account for the things I said during the talk):
Next 6 slides:
Last 7 slides:
Later, I will show specifically how the ice gets arranged into all these strange shapes.
My older sister sent me this photo on a recent morning.
This is hair ice growing from an alder tree. All of it grew overnight, formed when liquid water near the outer trunk surface (beneath the bark) froze. This creates an ice front in each pore-like structure. Water inside the trunk flowed out to the ice front, pushing each hair-like strand of ice outward.
It probably does not get as cold under bark unless the bark is partly peeled off, so the hair ice comes out of bare spots in the trunk, as shown in the photo above. The air temperature got down to 26 F that night, and the tree was next to a dirt road, and thus partly exposed to the sky.
Like the needle ice that grows out of the ground, it grows relatively fast, producing relatively long hairs. From another spot, down near a meadow:
Here's a reed of some sort, with a glaze of clear rime on the side facing a creek. How did the clear ice get there, and what is the white ice on top of it?
The fact that the clear ice is just on the side facing the creek indicates that the ice came from droplets blown off the creek. To have the ice buildup just on the side means that the droplets must have frozen soon after landing, but slow enough to spread out and fill gaps.
So, the droplet must have been larger than typical cloud droplets, and the temperature must have been within a few degrees of freezing. Clearly, the flowing water must have been above freezing, so the droplets must have cooled to below zero while drifting through the air.
A closer shot shows that the outermost ice, the white ice, is hoarfrost. Only hoarfrost would show the flat crystalline facets. Definitive proof that that ice grew out of the vapor that was blowing by. You can also see some hoar on the other side of the reed.
Cold air came down the interior, spilling through gaps in the Cascades, cooling the western side. In the Seattle area, we got our first frost on Nov. 11, but the days stayed cold. Areas in the shade never lost their hoar, so the hoar frost kept growing.
But does a hoar-frost crystal continue growing the same way as on the previous night? Does it keep extending uniformly, getting larger and larger, keeping the same general shape?
Or do smaller hoar crystals form on previously grown larger crystals?
A few days later, I found myself walking in Maple Falls near Mt. Baker. In a shady pocket near my feet, I saw some hoar. Subtly different -- it appeared like small, translucent leaves encrusted on their edges with hoarfrost. But it turned out that the "small leaves" were actually tabular hoar (plate-shaped). That is, hoar encrusted on hoar: smaller crystals on larger crystals.
When frost resumes growing on previously grown frost, the other possibility may also happen; that is, the hoar just resumes growing the way it did the previous night. But I think this is generally quite rare because it would require very slow and constant growth conditions, which is generally incompatible with the diurnal temperature cycle. See the sketch below for both possibilities.
Note that I write above 'smaller hoar crystals grow on the larger ones' even though technically, they are all the same crystal. You can see this is true by the fact that their faces all line up.
The view that got me to look closer:
In summer, the sun reaches higher elevations, bringing the possibility of new atmospheric displays. I saw this one in mid-June.
The top, upward arc is the more familiar and common 22-degree halo. But pay attention to the one on the bottom. Its colors look like a rainbow, but it has nothing to do with water drops of any sort.
It is the circumhorizontal arc, and it is as rare as it is beautiful. The colors are, in fact, more pure than those of the rainbow, and its origin is far more remarkable. It is remarkable because of the surprising set of conditions that must hold for it to exist. First of all, the particular region of cloud (at a certain angle to our eye) must consist mostly of ice crystals. Second, the crystals there must be nearly perfect tabular prisms. And third, these crystals must be in sufficiently non-turbulent air and of such a size that they fall in a nearly exact horizontal position:
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.
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.
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.
Morning on the spring equinox, the first day of spring, brought a few gifts from winter. I) Film-frost, accentuated with hoar-frost on the cars.
See the white of the hoar, following a pattern set down by the thin layer of film-frost. The roof of one car:
And the roof of another:
So it went. As I walked through our parking area, I saw a different pattern on every car - a pattern that told a tale of the night's weather and evening conditions. The film frost here is dramatic because the water film was thick before it froze. And it was thick because it had been raining in the previous evening. Later that evening, the skies cleared, and the temperature dropped rapidly, ensuring that the thick film would freeze. And the warm wet weather of the previous day left plenty of vapor to deposit as hoar-frost.
Car hoods had their own story to tell:
Take about a cup of water, place in a glass flask, then plunge in liquid nitrogen. Watch.
What happens in that video is that the water starts to freeze from the bottom and sides of the flask, and the ice also spreads across the top surface. When the water turns to ice, it expands by about 10% and so the remaining water must be pushed aside. If a hole remains in the top ice layer, the water gets pushed out the top.
What happens next depends a lot on various, poorly studied features of the situation. But sometimes, the out-flowing water forms an ice spike like that shown above. Sometimes it doesn't. In a previous post (02/06/2011), I describe a vase-like shape that formed in an outdoor bathtub:
The trick of putting the flask in liquid nitrogen (about -200 C) came from Larry Wilen back in about 1989 when we were at the University of Washington. I later did the trick a few times at the University of Arizona, but I thought I'd try to perform it during a talk I gave in January, 2013. So, I went to see the chemistry-department demonstration guy, Eric Camp, at the UW, and we tried to replicate the demo. Now, in the past, I could get the spike pretty much every freeze, but Eric and I had the hardest time with it. So, in the end, I got the video, but didn't attempt the stunt at my talk.
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.