Chelsea Whyte

The shape of a snowflake

It’s long been known that different temperatures influence the patterns of ice crystal growth. Near -2 °C, they grow into small, flat discs. At -5 °C, they form slender columns and needles. Near -15 °C, the thinnest, largest flakes, like the one pictured, emerge. At temperatures below -30 °C, they make columns once again.

So what makes snowflakes like this one grow so large and flat? In his recent study, Caltech physicist Kenneth Libbrecht found instability in their growth that leads to what he calls a “sharpening effect”.

As the snow crystal develops at -15 °C, a small ridge builds up at the edge and then spreads out toward the moist air in a sharp blade. Because the corners of the hexagon stick out farther than the centre, they collect moisture and grow faster.

“As soon as the ledge gets a little bit sharper, then it grows faster, and if it grows faster, then it gets sharper still, creating a positive feedback effect,” Libbrecht says.

This is just the first step in snowflake science. Libbrecht says he still doesn’t know exactly why different temperatures lead to sharpening on different faces of snow crystals, resulting in columns, discs, and plates.

Chelsea Whyte

Image – snowcrystals/ newscientist

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