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):

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

[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.

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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[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 (but does NOT cause) 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).

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.

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

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 "Not in fresh, light snow, though possible in more compressed or wetter snow." 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.)

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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.

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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[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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[This is the third of the re-posted articles, from 2008.]

For most of his life, and while not attending to farm duties, Wilson A. Bentley was captivated by the beauty of snow crystals. Perhaps then it should not be too surprising that he pondered the physical cause of their symmetry and intricacy. His thoughts turned to electricity as an explanation, perhaps influenced by his era's popular-culture infatuation with the new-fangled electrical devices. Though erring in the details, he was clearly onto something as I will show.

He considered that the crystalline surfaces had electric charges, with more charges concentrated at branch tips (1). When the tips of the branches overflowed with charge, the charges dribbled down the sides to produce ‘growth nuclei’ for the sidebranches. This process, he argued, could explain the symmetry of snow crystals:

"That the crystals, when permitted, attain to such a marvelous degree of symmetry and complicity, shows that the alignment of the growth nuclei, presumably tiny electric charges, is symmetrically regular to an almost unbelievable degree."

Here he connects electricity to the formation of sidebranches and the branch symmetry. In other writings, he connects snow electricity to growth rate, and snow electricity to lightning. In the specific details he was wrong, but in general he was surprisingly prescient. Snow crystals are indeed electric crystals, and the electricity itself is captivating. To see why, consider some of the amazing things that Mr Bentley’s electric crystals can do.

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[From 2006 through 2012, I contributed annual articles to the annual newsletter "Snow Crystals" for the Wilson Bentley Historical Society. That newsletter is no longer available, so I will repost my articles here, starting with this one from 2006.]

Wilson Bentley is well known to readers here for his photomicrographs of snow crystals. Snow, however, was only one of the many ‘water wonders’ that held his fascination (ref. 1). Some of these wonders were made of liquid water, such as dew, and some, like the snow crystal, were frozen water (ice).

The frozen type he called “The snowflake’s closest of kin”, and they included hoarfrost, rime, windowpane frost, and ice flowers (2). To obtain photographs of any of them with the quality obtained by Bentley is difficult even now, which is yet another reason to admire Bentley’s skill and perseverance.

On the inside, these ‘kin’ all have the same crystal structure. But they appear different on the outside, largely due to the different ways the water in the surroundings gets to the ice surface. There are many distinct kin because the surrounding water can be in various states (i.e., ice, liquid, and vapor) and there are many ways that each state of water can get to the ice surface. I’ll focus here on snow crystals, hoarfrost, rime, windowpane frost, and ice flowers. These forms are commonly seen by many of us, and have been observed by people for a very long time. So it is easy to think, as I probably once did, that they are well understood by science. But this view is quite mistaken. Yes, we know they all consist of H2O molecules and we know something about the structure of ice, but how exactly they form contain many mysteries. I’ll describe briefly what Bentley thought of them, and what I think is known and not known about them.

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Out walking one day last spring, I sensed a subdued illumination--like a thin veil of cloud had drifted in front of the sun. I looked up, and indeed found that there was. Blocking out the sun with my hand, I surveyed the thin veil and saw a strange sight: instead of the usual 22-degree halo around the sun, I saw what looked like two closely spaced haloes near the usual 22-degree spot. I took a picture:

(My hand is blocking the sun.) Clearly, at least one of those haloes were uncommon. But which one (or ones) and what was causing it?
Below, I mark the positions of the observed haloes in green and in black the position that the common 22-degree halo should be.

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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.

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.




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

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