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How some snow-crystal centers retain their droplet origin
If you zoom in on the center of some snow crystals, you may see a small circle, or near circle. In some cases, the crystal will have other circles of similar size as well, but the one to focus on is the one at the exact center of the crystal. Here marks the spot where the crystal was born.
Look at the center of this one (one of Mark's crystals):
The question is, what is that circle in the center?
The answer is: It is the original droplet upon which the crystal first formed.
Now that is the answer that I had heard a long time back. The originator, I think, was the incredible Wilson Bentley, the farmer-scientist of Jericho, Vermont during the early 1900s.
The problem was that it was never clear how the crystal could build upon the frozen droplet without erasing its boundary. We see this all the time in the lab - the droplet freezes, and then the vapor molecules add onto the crystal, turning it into a hexagonal prism, and then growing into whatever shape the conditions and crystal structure determine. There are absolutely no markings, borders, or interfaces that show what the original frozen droplet looked like. It is like pouring water into a cup that is half full: after you pour a little, you cannot tell that the cup once had less water. So, I never understood how Bentley's answer actually worked. Indeed, I think that until Akira Yamashita came along and pointed out how the boundaries of the original droplet could remain, nobody knew. His argument, which he proposed just a few years ago, involved the same curious process that we found in the corner pockets (see the previous post).
The process is sketched out in A-D below:
The frozen droplet under "a)" spreads an ice layer mainly along the top and bottom, making the cap and boot in 'b)". The molecules spreading across the flat surfaces migrate over the edges to the adjacent rim region, and plug into growth sites there. So the cap and boot spread out beyond the rest of the frozen droplet, as in "c)". The sides of the frozen droplet are essentially in a "vapor shadow" and receive almost no vapor: they hardly grow at all, and remain essentially frozen at their original size.
This process of the top and bottom (cap and boot) levels growing outward continues, making the crystal look like the empire fighter ships in Star Wars.
So, "c)" shows the two levels of every snow crystal. But the top and bottom levels are too close to each other. If one gets ahead of the other, it leaves the smaller one in a vapor shadow, just like the sides of the original frozen droplet. These other Japanese physicists found out how one side gets favored: It is all in the fall orientation. Remember that snow crystals are falling through the air, even as the air pushes them higher. Small, flat objects tend to hang in the air broad-wise, like a frisbee or flying saucer. The lower side (boot) gets a little more vapor, being upwind, and robs the top side (cap) of its fair share. So the lower level gets much bigger. Later, this frisbee flips back over, but it is too late for the stunted level: it remains forever stunted as the other level takes off, growing the six main branches, side-branches, and whatever features develop on its long journey to the ground.
But these stunted regions remain, including the sides of the original droplet. Not all snow crystals have this center. In many, the droplet gets erased as we observe in the lab. More on that in my next post. If you read this far, you now know something about snow that essentially nobody else does.
Finally, about those other circles on the crystal, the ones that look similar to the one in the center. These too are frozen droplets, droplets that had a startling run-in with the crystal and froze on contact. When they do this, we call them "rime".
(Crystal image is copyrighted to Mark Cassino, who let me repost here. The Bentley excerpt is from a poster I made years ago. Please do not reproduce without permission, thanks.)