Your interpretations of how to classify the snow crystals make a lot of sense, Jon. And thanks for the kind words about the first image in the post –the snow can fall in beautiful crystals when conditions are right, but more often than not I see rime cover and fluffy white, opaque crystals. The clean and translucent ones are a rarity.

- Mark

I have a hard time taking my eyes off the first crystal - such a clean and sharp image!

So, I came up with a few different ones than you. One difficulty is in choosing the underlying pattern. For example, is the underlying pattern in #1 dendritic or stellar? You thought the former, I thought the latter.And then there is a grey zone between ordinary dendrite and fern-like. Another difficulty is distinguishing sectorlike from plate extension.

As I read things, a dendrite should have some significant sidebranches, whereas a stellar should have no major sidebranch.

About sectors, I've never seen anyone really clarify the meaning, so I've taken it to mean segments that have almost a pie-slice shape. I didn't see this in #1, so I voted for P2a, stellar with plates at the end. It is true that the plates have some small sidebranches at the ends, but they seem like small additions at the end of growth so I ignored them. (Stunning though!)

On #2, I also went with plates at the end, calling it P2c - dendrite with plate ends.

On 3,4,5&6, I got the same as you: R1d, P4a, P1e, and P1e.

#7 seemed close to fern-like, so I voted for P1f. But it is a tough call.

#8 is a toss. I figured it could be either R2b or R1d. I leaned towards the former though, and you picked the latter.

#9 & 10 I got the same as you: R2b.

Interesting view about the classification with all the rime. They must explain the choices in their 1966 article. I have the article somewhere in my collection, but can't find it.

I suppose though that they wanted the classification to give useful knowledge about the cloud conditions. Rime is a very useful observation because it tells us that the cloud had a lot of liquid water (i.e., many drops). The underlying shape is important too, but harder to interpret, even now. Presently, we don't have enough knowledge to be able to infer the humidity and temperature except in special cases, like the fern-like form. For example, broad-branches can form at two temperature zones.

Jon

Thanks, Jon - I have more to post and will see if I can Classify them...

- Mark

That first one is hard to classify. I suppose it fits best into P2g - plate with dendrite extensions. Very pretty and very symmetric.

The second one - a nice rimed stellar R1d, I like it. I've never noticed how a little rime can make a crystal look even nicer. It must be the skills of the photographer!

And the third one is another R1d - a rimed stellar. It is quite different from the second one though - so it seems like we need some intermediate classes.

Jon

Lots of people have ideas about this, but no one really knows exactly how these patterns form (there are probably lots of ways, really). I just finished a dissertation on this kind of pattern formation in small molecule crystals, and I think it's safe to say that there is no definitive mechanism that applies to every system. (Look up banded spherulites if you want to read more...it's a common morphology in polymers and small molecules, and has been studied for well over 200 years.) In this case, it is very likely that the crystals grow via "rhythmic deposition," very much like you suggest. I would guess that there's a thin film of water on the windshield, and that a small polycrystal nucleates and grows until it uses up all of the water close by, and then has to wait for more water to diffuse into the gap it has created. Since some crystals will have reached further out from the center of the polycrystal than others, they will begin to grow first, creating a new ring with an empty space left between the new ring and the old.

As far as your dark spot is concerned, the seed for the polycrystal can be very, very small, but once growth begins in the liquid film, it progresses very rapidly outward. The central seed doesn't have time to grow very big before it is surrounded by new crystals that use up all of the available water. Also, if the film is very thin, the seed can pierce the surface of the liquid. Once broken, the liquid will pull away from that central spot due to the surface tension of the water, thus leaving an empty hole.

In any case, your mechanism sounds just about right. Even though the center looks dark, I would bet that if you could look very, very closely you would probably find a tiny seed that started it all.

Edvard Munch in the frost ;-)

Thanks Mark.

Too bad there wasn't a circle cut in half under the eye. When I find that one, we'll have "frost scream". Actually, that's a bit hard to say.

Eyes of Frost is one cool photo, Jon. Great!

- Mark

Thanks for the link, Mark. I loved the book! Now, whenever it starts snowing, I look at the tiny flakes and imagine what they look like up close. (I haven't managed to catch any to actually view up close... yet!)

The center part of this crystal is amazing - the most beautiful I've seen.

Wow - you live in a really cool neighborhood, Jon! I doubt that a bull has stepped foot anywhere near my house for well over 100 years.

Beautiful photos and nice essay.
About making observations, even though I live in a suburban area, I find that if I go outside for a stroll (always with my camera), I always find something.
Yesterday there wasn't much ice that I could find, but on my way back I discovered a black bull grazing in someones (slightly overgrown) parking lot. I never would have expected it.

Wow Mark, those are fantastic.
The top one, which as you said, looks like two crystals, might be a case where even the normally 'stunted' level of the crystal developed significant branches. I haven't really studied the levels on many crystals, so I don't know how rare this is. It is hard to discern the second level on the bottom crystal.

I don't know much about these spiders, but the ones last year that became active again after a freeze were still on their webs. After this light snowfall, the spider near our front door (also with a few legs missing) hung on for a few days longer, but it too is gone now. Maybe they can't survive very long in the cold.

This is really interesting - I wonder if the cold got to it of if it 's bilogical clock had just run out? At any rate, good observation to find this.