The Natural Enquirer: More about snow
I think I was a little negative in my last article. Sorry.
It turns out that “pure” snow is made up of snowflakes, which are made up of from two to 200 separate snow crystals. Snow crystals are crystals that have formed around tiny bits of dirt that have been carried up into the atmosphere by the wind. So snow crystals are really soil particles that have been dressed up in ice.
Scientists think that there are really five different shapes of snow crystals. The simplest shape is a long needle shaped like a spike. The other shapes all have six sides. One of them is a long, hollow column that is shaped like a six-sided prism. There are also thin, flat six-sided plates. The most beautiful are the intricate, six-pointed stars. We will include grapple, which are seen as very small, snowball-like things looking like miniature soft hail.
The shape that a snow crystal will take is dependent upon the temperature at which it was formed. The temperature in the highest clouds is around -30 degrees Farenheit, and they are made up exclusively of ice crystal columns. The other three shapes are formed in a narrow temperature range. When the temperature in the clouds is 3 to 10 degrees, the star shaped crystals form. From 10 to 18 degrees the plates form, and from 18 to 23 degrees, columns form. From 23 to 27 degrees needles form, and from 27 to 32 degrees the plates reappear.
As the snow crystals grow they become heavier and fall towards Earth. If they spin like tops as they fall, then they may be perfectly symmetrical when they hit the Earth. But if they fall in a sideways fashion then they end up lopsided. Falling snow crystals clump together forming snowflakes.
One thing you notice right away about snow crystals is that they form some elaborate and complex shapes-often displaying lacy, branching structures. Where does this complexity come from? After all, snow crystals are nothing more than ice which has condensed from water vapor. How does the simple act of water vapor freezing into ice produce such intricate designs?
The answers to these questions lie in just how water molecules travel through the air to condense onto a growing snow crystal. The water molecules have to diffuse through the air to reach the crystal, and this diffusion slows their growth. The farther water molecules have to diffuse through the air, the longer it takes them to reach the growing crystal.
So consider a flat ice surface that is growing in the air. If a small bump happens to appear on the surface, then the bump sticks out a bit farther than the rest of the crystal. This means water molecules from afar can reach the bump a bit quicker than they can reach the rest of the crystal, because they don’t have to diffuse quite as far.
With more water molecules reaching the bump, the bump grows faster. In a short time, the bump sticks out even farther than it did before, and so it grows even faster. Small bumps develop into large branches, and bumps on the branches become side branches. Complexity is born. This instability is a major player in producing the complex shapes of snow crystals, and since the ambient atmospheric conditions are nearly identical across the crystal, all six budding arms grow at roughly the same rate.
The temperature seen by the snow crystal is not constant in time, however, since the crystal is being blown about, and is thus carried over great distances in a cloud. But the crystal growth rates depend on temperature. Thus, the six arms of the snow crystals each change their growth with time, reflecting the ever-changing conditions in the cloud. And because each arm sees the same conditions, each arms grows the same way.
When the branching instability applies itself over and over again to a growing snow crystal, the result is called an ice dendrite, (dendrite means “tree-like),” and stellar dendrite snow crystals are common.
We can change diffusion in the lab and see how dendrites change. If one grows snow crystals in air below atmospheric pressure, they have fewer branches. This is because diffusion doesn’t limit the growth so much at lower air pressures, so the branching instability is not so strong. At higher pressures, more branches appear.
The growth of snow crystals depends on a balance between temperature and humidity.
Faceting tends to make simple flat surfaces, while branching tends to make more complex structures. The interplay between faceting and branching is a delicate one, depending strongly on things like temperature and humidity. This means snow crystals can grow in many different ways, resulting in the great diversity we see in snow crystal forms.
So that’s the story. The intricate shape of a single arm is determined by the ever-changing conditions experienced by the crystal as it falls. Because each arm experiences the same conditions, however, the arms tend to look alike. The end result is a large-scale, complex, six-fold symmetrical snow crystal. Since snow crystals all follow slightly different paths through the clouds, individual crystals all tend to all look different.
And a short poem from one of my Nature Center exhibits;
Snowmen small and snowmen tall
Each formed from flakes by hands so small
Loved by you and loved by all
Peter Dring is a retired nature biologist and phenologist who lives in the Land ‘O Lakes area. Questions or comments for Dring can be sent to [email protected]