Ask meteorologists: how do ice crystals form and grow? | Weather

A: This question was considered by astronomer Johannes Kepler about 400 years ago.

Kepler published a paper on the subject in 1611. He hypothesized that crystals were made up of subunits that combined to form the symmetrical shapes of ice crystals.

Perhaps the person best known for studying ice crystals was Wilson A. Bentley, a Vermont farmer. He captured over 5,000 exquisite photographs in his lifetime. His photographs show that ice crystals are all six-sided. Their beauty echoes the complexity of their formation and growth.

Physical chemistry explains geometry. Ice crystals are made up of water molecules, which are made up of two hydrogen atoms and one oxygen atom. The two hydrogen atoms form an angle of 104.5 degrees with respect to the atomic nucleus. The oxygen atom attracts a greater share of electrons, which makes the water molecule slightly negative on one side and slightly positive on the other. When water freezes, the bipolar molecules are attracted to each other, forming a hexagonal crystal lattice.

When ice crystals form, water molecules cannot drop onto the crystal randomly. The molecules must match the shape of the crystal.

The shape of a crystal is called its habit. As Bentley’s photographs show, there are four basic habits of ice crystals: the hexagonal plate, the needle, the column, and the dendrite.

The temperature and vapor content of the environment determine the particular crystal habit. As the crystal moves through the atmosphere, the temperature and humidity change, which can alter the growth habit and produce very complex shapes. The final shape of the crystal will vary depending on the environmental factors it has experienced while passing through the atmosphere.

“Weather Guys” Steve Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at the University of Wisconsin-Madison.

About Jefferey G. Cannon

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