Why are snowflakes hexagonal like the cells of a honeycomb? Are they made by ice bees living in the clouds? Not quite. But, both in the sky and on Earth, the laws of physics are the same.

In preparing to describe snow and how it is formed, let’s put ourselves—obviously only in our imaginations—inside a cloud. At the same time, let’s stick to the scale of the atom, from which we can observe the behavior of individual molecules of water.

Water is dipole. This means that we can differentiate between the positively and negatively charged ends of the molecule. We know that minus is attracted to plus, so molecules of water tend to clump together. The colder they are, the slower the molecules vibrate, which increases the chances of dipoles grouping together. The atmospheric conditions of a cloud, above all its low temperature, encourage the creation of water molecule groupings.

In order for snowflakes to form, however, we need an impulse.

The appearance of a speck of something, like dust, pollen or bacteria—i.e., a nucleus of condensation—is the impulse that initiates crystallization in a supercooled swarm of molecules. There are methods for the artificial crystallization of clouds, including the atomization of silver iodide particles. The thing that