Snow Crystals and a Penny

Kenneth Libbrecht, Professor of Physics at CalTech, collects snowflakes and grows his own.   Here he compares them in size to a US Penny.   “The largest crystal in that image is about 1 cm from tip to tip. I believe it still holds the record for the largest individual snow crystal ever photographed.

See his books “The Secret Life of a Snowflake”,  “Field Guide to Snowflakes” and more.

All images ©Kenneth Libbrecht, shown with permission
Some 'simpler' crystals, hexagonal prisms and prisms capped at each end by hexagonal plates.

Snowflakes start their life in a a cold cloud. Its small water droplets are below zero, supercooled.   The supercooled state is metastable. If a droplet freezes then its vapour pressure is lower than that of the surrounding drops. The crystal grows and surrounding drops evaporate to feed the growth.

The growth is not simple and regular hexagonal prisms rarely reach a large size – when they do we see halos. More often, instabilities set in. The edges of facets grow more rapidly forming dendrites. Dendrites grow their own dendrites to give the beauty, symmetry and intricacy of a snow flake.

What happens depends on many factors like humidity and temperature. For more about it see Libbrecht’s review of snow crystal physics.

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Ice crystals reflect the underlying hexagonal structure of solid water.

In this chemists' 'ball and stick' model, oxygen atoms are red.

Between each, but not equidistant, is a hydrogen atom. The bonding of each hydrogen to two oxygen atoms generates an open, low density, structure to ice. That's the reason ice floats on water. A factor to which we owe our existence.