Curved Film Frost, Part 1: On the General Causes

March 15th, 2022

As I've seen it, the three most common ways to depict coldness in art and culture are the snow crystal, the icicle, and the curving frost on windows.

Curved Film Frost, Part 1: On the General Causes

All of these have seen relatively little study, but without question the last has received the least. Indeed, I know of only three studies dedicated mainly to curving frost, and all three are over 50 years old.

It is perhaps little known that in terms of being observable to the typical observer, the third would likely be far more common. An odd thing to say? Well, people may say that they see it far less now due to the common use of central heating, or some might say that global warming is making it harder to form, but in these arguments they are wrong. I think the opposite: it is in fact far easier to see it now than maybe anytime in the past due to the ubiquitous presence of cars parked outdoors.

One curious thing about curved frost is its defining feature, the curviness. This feature contrasts with the straightness (and regularity) of snow crystals. What is the cause of the curviness in the former, as contrasts with the latter?

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Avalanches, Part 2: Effect of Debris

March 5th, 2022

In part 1 (1), motivated by an avalanche story, we saw that avalanching snow would hardly increase its temperature from all the tumbling. In fact, it would, in falling down some elevation X, warm up by an amount that would be about half of that of the increase in local air temperature at the lower elevation. So, in a sense, you might even argue that the avalanching snow has cooled relative to the surroundings. Even more surprising, you might think, is that even if the snow had been at the melting temperature before avalanching down, only a tiny fraction of it would melt, even if falling down 1000 m.

However, going back to a story about the debris-filled avalanche in the Himalaya, consider now a boulder tumbling down a mountain. It hits some snow and comes to rest. As with the tumbling snow case, the energy the boulder had above will transform to heat energy when it comes to rest. How much snow will melt?

A maximum possible amount can easily be estimated by assuming that the tumbling does not slow the boulder, the boulder itself does not heat up, and the snow is already at the melting point. The amount of melt will be in proportion to the size of the boulder, so we look at the fractional volume of snow that melts (volume of snow divided by volume of the boulder):

Avalanches, Part 2: Effect of Debris

As you can see from the table, the boulder is more effective at melting the snow than just the tumbling snow case considered in (1). This result seems reasonable given that a boulder is denser than ice and thus has more potential energy. Even so, relatively little ice will melt.


The reason for the latter is the large amount of energy needed to melt ice. For example, to melt a chunk of ice that sits right at the melting temperature equals the amount of energy needed to warm the ice from a much cooler temperature to the melting point.

How much cooler, you might ask?

Well, about 285 degrees Fahrenheit (159 degrees C). So, if you are out in the winter and need to melt some snow at -20 C, and you've used half your stove's gas just to get it to -1 C, you might think that you are nearly there. But in fact you will need to get a lot more gas for your stove before you can even think about melting all of it.

--JN

References and notes

1. Avalanches, Part 1: Snow Hardening
https://www.storyofsnow.com/blog1.php/avalanches-part-1-snow-hardening

Frozen Drops on Web

March 4th, 2022

They condense from the vapor, get jiggled and jostled, coalesce, and then suddenly, they freeze.

Frozen Drops on Web

You can see a few strands, more nearly vertical, that must have had a drop slide down soon before freezing, sweeping out all the other droplets and leaving only a thread of densely packed tiny drops.

The freezing, one started, likely spreads along a thin film on the web from drop to drop, freezing all drops. The largest drops tend to be at thread intersections that tended to keep them from moving further. Other droplets may have slid along, only to collide and coalesce with the one fixed to several threads.

The spider made mostly four-sided polygons, though in the center of the web, you find more variety. Some triangles, some pentagonal, some hexagonal.

In the atmosphere, we call just-frozen droplets, having only enough vapor growth to imagine facets, as droxtals. Perhaps some of the smallest ones here could be droxtals.

--JN

Bentley’s Most Singular Observation

March 2nd, 2022

[This is the seventh and last in the series of re-posted articles, from 2012.]

You don’t have to look at frosted surfaces for very long before coming across something like the following.

Bentley’s Most Singular Observation

The picture shows a large ice crystal amid a roughly uniform sea of tiny frozen droplets. Between the large crystal and the frozen droplets lies a clear ice-free zone, a dry moat around an island of ice. Sometime prior to 1907, the Vermont farmer Wilson A. Bentley took notice of this moat. Writing in the Monthly Weather Review in 1907, he wrote

"One of the most singular, and doubtless most important, phenomena that occur in connection with the formation of window frost is this: The true crystalline varieties of window frost ordinarily, apparently, repel the minute liquid particles or droplets of water that frequently collect like tiny dew-drops on the glass, and freeze in granular form thereon."

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Why six?

March 2nd, 2022

[This is the sixth in the series of re-posted articles, from 2011.]

Why do so many snow crystals look about the same when rotated by 1/6 of a turn? What’s the origin of the six-fold symmetry? Why not five or seven, as Kepler asked(1)?

Since it always happens, when it begins to snow, that the first particles of snow adopt the shape of small, six-cornered stars, there must be a particular cause; for if it happened by chance, why would they always fall with six corners, and not with five, or seven, as long as they are still scattered and distinct, and before they are driven into a confused mass?

Though we can now answer the last two questions, the first, as pondered by Bentley and many others, still awaits a complete answer.

The thing that allows the crystal to have six-fold symmetry comes down to in its internal crystalline lattice of water molecules. Specifically, if you could zoom in about a million times into any region of a snow crystal, such as the corner in figure 1A below, you would see a lattice of hexagonal rings B) – like a microscopic internal honeycomb. The oxygen atoms (black) in each ring have the six-fold symmetry. But if you further examine the rings, notice that all of them are rotated by 30º in relation to the crystal hexagon in A).

Why six?

However, if you look even closer, such that you can see the orientations of the molecules, and turn the ring on its side, as in C), you will see that in fact the ring is not perfectly six-fold symmetric and it’s not even flat! The honeycomb inside ice is not so simple. But if you sought the overly simplified answer to the origin of the “6”, find it in the hexagonal lattice. Call that the answer for the easily persuaded. There are serious problems though with that answer.

The simple answer specifically has two problems. The internal honeycomb just gives us a similarity between the ice lattice and the crystal form – it suggests that the crystal may develop six-fold symmetry. But just because a crystal may have six sides doesn’t mean it will have six sides. Or, to use one of Kepler’s examples, a beehive also has an internal honeycomb structure, and yet from the outside it appears blobby and nondescript – hardly six-sided.

Why six?

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Blues and Whites of Snow and Ice

March 1st, 2022

A recent article in the local newspaper asks the question "Why does snow glow blue?"

Blues and Whites of Snow and Ice

The author gave one inspiration for the article as "..the way white snow glows turquoise in the holes left by boots or ski poles..."

The explanation given in the article is the same as that you can find elsewhere on the internet, and certainly applies to glacial ice: absorption of the red end of the solar spectrum upon passing through ice. Does it indeed apply to "holes left by boots or ski poles" in snow?

(The answer is no, and for some quirk of this blog software, I cannot put the following plot in its proper location further down, so I place it here. Please ignore it for now and refer to it at the end of this post.)

Blues and Whites of Snow and Ice

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Avalanches, Part 1: Snow Hardening

February 25th, 2022

Why does snow seem to harden after an avalanche?

Those unfortunate few who have been buried in an avalanche often observe the snow, once stopped, has hardened. Some liken it to concrete. Whereas the powder before the avalanche seemed light and easy to move, once buried, the victim finds it impossible to move.

I have never had that experience, though I find the observation reasonable. For example, on a recent hike up a snow-covered hillside, I used snowshoes to avoid "postholing" through the snow. But when I came to a region where snow had slid down, I removed the snowshoes and easily walked on the surface.

Avalanches, Part 1: Snow Hardening

You can see in that picture that considerable debris was present in that case, though it is unimportant for the present discussion.

An article I read last year about a similar event, yet on a vastly larger scale, reported quite the opposite: the ice melting by its tumbling fall down the mountain. Here is the relevant excerpt:

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Ice Classification System of Bentley

February 24th, 2022
Ice Classification System of Bentley

[This is the fifth in the series of re-posted articles, from 2010.]

The most comprehensive study of frost and small ice formations was published way back in 1907 by Wilson A. Bentley. His article was split into five successive issues of the Monthly Weather Review (1) with the title “Studies of Frost and Ice Crystals”. In the article, Bentley details his classification system for frost and some related ice forms, and gives examples of nearly every type – in total 274 examples in photographs. His classification system is ingenious, the first (and probably only) classification system for frost and ice on surfaces. Unfortunately for such an extensive, pioneering article, few books or articles reference it.

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Snow Crystal Variety and the Bentley Length Scale in Clouds

February 22nd, 2022

[This is the fourth of the re-posted articles, from 2009.]

“… by means of these wonderfully delicate and exquisite figures, much may be learned of the history of each crystal and the changes through which it has passed in its journey through cloudland.”
W. A. Bentley, 1898

“… the crystals will in all probability be greatly modified by passing through atmospheric strata varying so greatly in density, temperature, humidity, etc. That they are greatly modified by these flights in the cloud is clearly shown by the interior structure of many of the crystals. ”
W. A. Bentley, 1901

Long ago, Wilson Bentley argued that the various layers (‘strata’) of air that a snow crystal falls through largely determine the appearance of the crystal. The first direct support of this claim came about 30 years later, from laboratory experiments by Ukichiro Nakaya, a Japanese physicist. Now, 100 years later, probably all cloud scientists would agree with Bentley. So, with all the crystal and cloud research that has been done since Bentley and Nakaya, can we determine the history of a crystal if we see its form? Can we finally do what Bentley said we should be able to do?

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Parting and Bending of Hair Ice

February 22nd, 2022

Hair ice often shows a clear "parting".

Parting and Bending of Hair Ice

By part, I refer to that line along the underlying log surface, either side of which the hair tilts further to the side, analogous to the part one makes in one's own hair with a comb. An explanation for the cause of the part is unlikely to be found in the nature of the pores (from which the ice originates), but rather in the differing rates of growth of adjacent hairs.

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