Snow crystals

How it grows

The crystal is grown with the lattice model of snow crystal growth that Janko Gravner and David Griffeath published in Physica D in 2008. Each cell of a hexagonal lattice, a few microns across, holds four things: whether it belongs to the crystal yet, a film of quasi-liquid, ice, and water vapour. Each update, the vapour spreads to neighbouring cells, freezes where it touches the crystal, and a cell joins the crystal once it holds enough.

The thresholds make the shapes. A cell at a tip or on a flat face, with one or two crystal neighbours, needs more to join than a cell in a hollow with three. Tips reach further into the vapour and grow faster, which makes branches; the higher threshold on flat faces keeps edges straight, which makes plates. Where the vapour runs thin between arms, a thin sheet can still spread, and that fills in plates long after the arms have run ahead.

The cloud chart covers −10 to −20 °C, where snow grows as thin plates. Following the Nakaya diagram, the wettest air near −15 °C grows the most branched crystals and the fastest, while drier air and the edges of the range grow faceted plates. Moving through the chart changes the vapour around the crystal and the rates at which its edge freezes and melts. Where each habit sits on the chart is this page's placement of the paper's parameters, not a measurement.

Rheinberg light has a blue stop in the middle and an amber ring around it: flat ice passes the blue, and slopes in the ice bend the ring's light into view. Dark field shows only the light the crystal scatters, and bright field only what it bends away. Vapour shows the air the arms have drawn thin, and timeline colours the ice by when it froze, in the same colours as the path on the chart.

A cell stands for 5 µm and an update for 0.03 s, the paper's order of magnitude. The microscope speeds growth up so each crystal takes about a minute to watch.