In the air, vapor’s swirling,
On the pond, folks are curling,
The vapor makes drops, the drops freeze and pop,
And six-sided snowflakes fall down.

On the lake, skates are gliding,
Overhead, clouds are hiding,
Ice in the sky is growing, oh, my,
And six-sided snowflakes fall down

Snowflakes could be square or five pointed,
Or octagons, or spherical, you know,
But water with water is jointed
So that only six arms can grow.

On the slopes, skiers swish on,
Snowflakes hide stars to wish on,
They fall through the air, and catch in your hair,
The six-sided snowflakes fall down.

The rhyme above can be sung, to the tune of “Winter Wonderland.” But it’s also a fairly good outline of why snowflakes look the way they do.

A water molecule is made up of one oxygen atom and two hydrogen atoms. The hydrogen atoms are not in a straight line with the oxygen atom, but are angled, like a bent line with the oxygen at the bend.

Ice, being crystallized water, is made up of water molecules in three-dimensional order. The water molecules in an ice crystal are held together by what are called hydrogen bonds — each hydrogen atom links not only with the oxygen in the water molecule, but with the hydrogen atom of a neighboring molecule. Given the shape of the water molecule, the easiest way the molecules can form an ordered structure is a hexagonal lattice. I’m not going to try to draw it, but there is a good drawing in this reference.

Most snowflakes actually start out as water droplets in clouds. A few droplets encounter ice nuclei as the temperature drops below freezing, and freeze into ice droplets. Sometimes the droplets explode to make many ice particles as they freeze, and each bit of ice can nucleate another droplet.

If ice and water are side by side at subfreezing temperatures, the ice will suck up water vapor from the water. The growth on the ice will be strongest at the sites where the crystal lattice juts out farthest, so the frozen droplet rapidly grows into something like a very short bit of a hexagonal pencil. The edges and corners of this hexagonal prism grow fastest, and sometimes even sprout arms.

Why are snowflakes often symmetrical, but different from each other? The type of growth is determined by the temperature and moisture of the air at the moment of growth. As each snowflake follows a slightly different path through the cloud, it will encounter a different sequence of growth than any other snowflake. At the same time, all of its six arms see the same sequence. The result is a snowflake that is fairly symmetrical but different from any other snowflake.

Very simple snowflakes – usually simple hexagonal plates or needles – may look very similar to each other. But the more complex dendritic snowflakes are generally one of a kind, because each has had a unique path through the cloud that spawned them.

We have snow of the ground now, here in Fairbanks, and many other areas farther south will soon. If you live in snow country, invest in a small hand lens and enjoy the myriad shapes of the snowflakes.

(Photos are from Bentley’s collection of snowflake images.)