I went on a skiing trip (well I snowboard) with my family. Below is a picture of my brother and I getting ready to snowboard down the hill behind us. Although it is hard to see from the picture, it was snowing! The slopes had a nice layer of powder snow on them. In this post I will take the opportunity to talk about the physics of snow. Welcome to #myphysicsjourney and #myphysicsvoyage!
The physics of snow starts with the movie called Frozen (from motivation by a Neil Degrasse Tyson Tweet @neiltyson). The movie takes us on an adventure to break the spell of the snow queen that causes the enchanted kingdom to be winter all of the time. The movie cover is shown below. If you were to zoom in on a snow flake as depicted in the Frozen movie cover, we see that it has six-fold symmetry. This means in the case of snow that there are six major edges or dendrites. If we were to rotate the snowflake by 1/6 of a full cycle it would look identical to the structure before we rotated it.
You may wonder why this structure is formed at all by water. It has to do with the molecular structure of water molecules. Water molecules are composed of 2 hydrogen atoms covalently bonded to an oxygen atom. However, a covalent bond is when an electron is shared by two atoms. In this case oxygen doesn't want to share the electron equally with the hydrogen atoms. This causes water to have a net polarization meaning that the oxygen atoms are slightly negative and the hydrogen atoms are slightly positive. Hence, when one water molecule comes in contact with another one, the oxygen atom in one of the molecules wants to stick to a hydrogen atom of another molecule. When six water molecules stick together they form a hexagonal structure. Combining several hexagonal water structures makes a snow crystal. For more of an explanation check out this blog post.
There is almost an infinite amount of variation in snow crystals (really they are polycrystrals in nature) because of the fluctuating and chaotic conditions under which they were created in the sky. The physics for how snow crystals grow in the sky is still not completely understood. The general physics involves understanding crystal growth in general. Statistical mechanics tells us that that the morphology of snow crystals depends on the pressure and temperature of the air along with the amount of excess water in the air. For a complete review see this paper. The next time you see snow, take the opportunity to observe all of the complex shapes that form! There is structure on almost every length scale.