I wanted to carve out a bit of time to share one more snowflake in 2019, and one that I think is the key to unlocking the physics of certain snowflake features that we still don’t completely understand.
A simple hexagonal outer footprint lends itself to some complex and puzzling internal features, such as the double layers we see near the very center. It looks as though the snowflake is growing a “ceiling” inward to cover a bubble at the very center! As crazy as that sounds, that’s exactly what is happening here.
Funny thing about snowflakes is that they don’t just grow outward. There are many examples of divots and sips in the surface of snowflake that gradually “fill in” as a snowflake continues to grow outward. See the trapezoidal shapes that show a contour from dark to light great all around the snowflake? Perfect example of this – if the snowflake were allowed to grow bigger, these contours would likely disappear as the surface would become more uniformly thick in these regions.
Something odd happens when these indentations in the crystal surface begin to fill in – if the edge between the “plateau” and the “valley” becomes sharp like a cliff, water molecules might only attach at the top edge of the cliff and grow from there, beginning to form a ceiling. This top layer of ice and the space directly below it can be so thin that at times it can evoke the phenomenon of “thin film interference” and produce colours in the snow. Previously, I thought these colours were formed by more traditional bubble formation: cavities on the outer edge that form because the center of the prism facets grow slower, and the outer areas grow outward faster to create bubbles in the ice. This is not the only way bubbles can form!
Here we have clear evidence that inward growth from a thick outer edge was echoing inward, becoming more rounded as it reached closer to the center. This is seen in the rounded hexagon shape seen just inside of the star shape in the middle. Imagine this is a cliff edge, which originally was a shallow transition but became steeper as it grow closer toward the center. My theory is that the closer these inward growth rings get to the center, the less likely it is that water molecules can attach to the base of the inward growth front and more likely that they attach higher up, creating a precipice from a shallow incline and eventually just growing out from a near-90-degree edge. I believe that’s exactly what we’re seeing here. :)
Wishing everyone a Happy New Year with plenty of discoveries in 2020!