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I want to know why snowflakes (and other crystals) grow symmetrically and I find the leading answer to the established question to be entirely unsatisfactory.

When water freezes, you get ice. Ice, like many solid materials, forms a crystalline structure. In the case of water, the crystalline structure may be attributed to the hydrogen bond, a special kind of an attractive interaction.

So a big chunk of ice will have a crystalline structure - preferred directions, translational symmetry, and some rotational symmetries.

Nature adds one water molecule at a time. The molecules always try to choose the most energetically favored position on the frozen body. Because these laws of creation of a snowflake are symmetric with respect to the rotational symmetries, it follows that any symmetry that exists at the beginning - a hexagonal symmetry of a small number of molecules in the initial crystal - will be preserved.

No, sorry, it doesn't follow at all. Something hugely important is missing from this answer -- causality.

enter image description here

Suppose the snowflake at one moment is perfectly symmetrical. Then suppose a random molecule gets added at point A. To maintain the symmetry, later on a random molecule gets added at a corresponding point B (and four others). But the motion of the water molecule is random and there is no reason for it to end up at B or anywhere else in particular.

Even considering theo's remarks about dendrite growth, why don't we see this?

enter image description here

I see absolutely no reason why any given additional water molecule cannot land in an energetically unfavourable position. Besides, if low energy considerations were a driving force, snowflakes would be approximately spherical.

So, what's really going on? How can a bump at A create an attractive force at a distant B?

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    $\begingroup$ It appears, after analyzing the various answers, that the "causality" you are looking for, is the photographer! By him/her "selecting" the most symmetrical flake to photograph, causes the rest of us to believe that "all" snow flakes are "highly" symmetrical, naturally. $\endgroup$ – Guill Jan 14 '15 at 22:27
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    $\begingroup$ I get that selection is happening, but brief calculations make me skeptical. Suppose we divide each of the main arms on my sketch into 10 segments, all points within each section being "equivalent" for the purposes of symmetry. The secondary arms have 10 candidate attachment points, and positing one secondary arm on each side of each main arm, gives $10^{12}$ permutations of even this basic shape. Did someone really search through that many to find the one in the photo? $\endgroup$ – spraff Jan 17 '15 at 11:32
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Broadly speaking, the reason why a snowflake forms a flat crystal with 6-fold symmetry (as opposed to a sphere) is due to a combination of the underlying symmetry (order) of the ice crystal and a dynamic instability (chaos) resulting from the non-linear phenomena of solidification and dendrite formation as a function of temperature and humidity variations in the atmospheric clouds.

Each snowflake is a crystal which grows out from a central nucleus of ice which has a 'hexagonal' symmetry. The simplest 'snowflake' is therefore a hexagonal prism.

enter image description here

When molecules of water 'adhere' to the surface of the crystal, the non-linear combinations of humidity and temperature create conditions in which flat surfaces are inherently 'unstable'. As the crystal tosses and turns in the atmosphere, water molecules solidify on tiny 'bumps' like the corners of the hexagon crystal so that these 'bumps' grow faster than the adjacent 'flat' regions, thereby forming long 'needle' like structures. As the 'needles' get bigger, any small irregularities along the edge become unstable and water solidifies on these to form 'branch' like structures. The tendency is for the needles to grow from the rectangular 'prism facets' rather than the hexagonal facets, thereby resulting in snowflakes with a flat shape rather than a 'spiky ball'. A detailed description can be found in this paper on "the physics of a snow crystal"

Because the snowflake is very small and temperature and humidity vary relatively slowly, the conditions for solidification of the water at a given point on the crystal will be the same at the 6-fold symmetrical points, so that the snowflake 'grows' symmetrically. In this way, smaller needles grow on previous needles to form a flat-plate with 'tree-like' structures (dendrites) around the a hexagonal prism nucleus in a 6-fold symmetric manner.

enter image description here

According to Professor Ian Stewart of Warwick University, author of "what shape is a snowflake?":

An ice crystal grows when molecules of water adhere to its surface. Certain combinations of humidity and temperature create conditions in which flat surfaces are dynamically unstable, an effect called the Mullins-Sekerka instability. In these conditions, if a flat surface accidentally develops a tiny bump, the bump grows faster than other nearby regions, amplifying the irregularity. A big enough bump is nearly flat, and becomes unstable for the same reason, so new smaller bumps proliferate. This process of repeated ‘tip-splitting’ leads to a fernlike pattern known as a dendrite. Dendritic growth causes the enormous variety of shapes seen in snowflakes, because the branching patterns are extremely sensitive to slight changes in humidity and temperature.

An lecture on the topic of mathematics of patterns and symmetry accessible to the layman can be found on Youtube. In this video (from minute 3:00 to 15:00), Ian Stewart gives an excellent introduction to the formation of symmetry within the snowflake.

enter image description here

A detailed description of the Mullins-Sekerka instability and the remarkable phenomena responsible for the formation of these beautiful patterns in snowflake crystals can be found in this paper by J.S. Langer.

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  • $\begingroup$ This doesn't really answer the question. The hole is this: "if a flat surface accidentally develops a tiny bump..." -- I can just about accept that, given that bumps promote future growth, symmetric bumps will lead to symmetric dendrites, but there is no a priori reason the instabilities should begin at corresponding points on multiple flat surfaces in the first place. Also, why should these instabilities create plates, which are flat-edged and unstable? This answer predicts a mass of dendrites. $\endgroup$ – spraff Jan 10 '15 at 11:28
  • $\begingroup$ In fact, if you pause here you can see the lack of left-right symmetry along a dendrite, presumably for this reason -- locations along either side of a needle are flat and equivalent, so randomness prevails. $\endgroup$ – spraff Jan 10 '15 at 11:33
  • $\begingroup$ Apparently, the 'simplest' snowflake is a hexagonal prism. The dendrites grow from the rectangular faces around the edge more easily than the hexagonal faces, so they tend to grow as a 'flat' crystal rather than a spikey 'ball'. Have a look at this paper which explains it this way: its.caltech.edu/~atomic/publist/rpp5_4_R03.pdf $\endgroup$ – theo Jan 10 '15 at 11:39
  • $\begingroup$ I thought the main question being asked is 'why the six-fold symmetry'? Although snowflakes generally exhibit this six-fold symmetry, most are not 'perfectly' symmetric, as explained here: its.caltech.edu/~atomic/snowcrystals/faqs/faqs.htm $\endgroup$ – theo Jan 10 '15 at 12:07
  • $\begingroup$ One part of the question is illustrated by this: consider a hexagonal nucleus with six equally long needle-like dendrites. A bump on one of these "parent" dendrites will lead to the growth of a child due to Mullins-Sekerka instabilities, okay, but why would a second bump appear on the opposite side of that dendrite? And why would pairs of bumps appear on the five other dendrites at corresponding distances from the nucleus? The other part of the question is why do plates form if nature prefers dendrites? $\endgroup$ – spraff Jan 10 '15 at 12:33
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But we do see exactly the kinds of asymmetries that you're asking about, even in your sample image. Here it is again, with some more letters and arrows:

annotated snowflake

If the ice crystals are more or less confined to a plane, you expect the crystal to be symmetric under rotations of 60º (one-sixth of a turn) and reflections about six axes in the plane. That's clearly approximately true for this snowflake. But look at the second biggish lobe on each long arm.

  • A reflection with lower-left to upper-right arm as its axis would swap $C\leftrightarrow D$ and $I \leftrightarrow J$, but those lobes are different sizes.

  • A reflection about the orthogonal axis would swap $C\leftrightarrow J$ and $D\leftrightarrow I$, but those lobes are different sizes.

  • A rotation by 60º clockwise would take $B\to A$, which as you point out are the same size, but would also take $(C,D)\to(E,F)$, which don't match.

If I had better image manipulation tools and a lot of free time I would overlay rotated and reflected images on each other to make the differences more obvious, but I think you should have the point. The asymmetries are there, but they are smaller than you expected. And there's a strong bias towards publishing photographs of exceptionally symmetrical flakes like this one. Libbrecht, who may have taken that image, puts it nicely:

If you think this is hard to swallow, let me assure you that the vast majority of snow crystals are not very symmetrical. Don't be fooled by the pictures -- irregular crystals (see the Guide to Snowflakes) are by far the most common type. If you don't believe me, just take a look for yourself next time it snows. Near-perfect, symmetrical snow crystals are fun to look at, but they are not common.

Elsewhere:

Even on the best of days, I search for hours to find just a few beautifully symmetrical specimens. I typically glance over thousands of crystals on my collection board before selecting one to photograph, and the pictures you see in the Galleries are some of the best among over 7000 pictures I've taken.

That's been my personal experience looking at actual snowflakes as well.

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  • $\begingroup$ Thanks but these are not the asymmetries I am asking about. You are pointing to different sized dendrite growths, I am asking about different locations of dendrite attachment. $\endgroup$ – spraff Jan 10 '15 at 20:58
  • $\begingroup$ But those are present too, just smaller than in your illustration. For example count the number of dendrites (including the little nubs) between $E$ and the rightmost tip and between $F$ and the same rightmost tip. $\endgroup$ – rob Jan 10 '15 at 21:22
  • $\begingroup$ Yes, I pointed out similar asymmetries earlier. The mystery is why such asymmetries are present at smaller scales but not larger ones. $\endgroup$ – spraff Jan 11 '15 at 16:48
  • $\begingroup$ Larger asymmetries are usually present too; edited. $\endgroup$ – rob Jan 11 '15 at 18:07

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