As we discussed briefly before, as water changes its state to ice, water molecules stack together forming a chrystalline lattice. This lattice has hexagonal symmetry which, in the end, is the determinable factor for snow crystal symmetry. The only problem with this description is the fact that the molecular forces that create these lattices are deciding the shape of a snow crystal that turns out to be about ten million times larger than the molecule itself. To diminish this problem, we consider the idea that crystals form facets. The facets on a small round ice crystal has many dangling chemical bonds, making the surface very rough. However, the water molecules present in
the atmosphere easily attach to these dangling bonds and the lattice grows quite quickly. But snowflakes lucked out with their facets, as their surfaces are generally much smoother on a molecular scale and have little, if any, dangling chemical bonds. ALAS, there is yet another problem with the facets in that the water molecules do not attach easily to the smooth surfaces, meaning it takes longer for the crystals to form.
So, after all of the rough surfaces have grown out, (see diagram below) the slow-growing facet surfaces remain. To put this all in simpler terms, the rough-surface facets grow quickly until all of the dangling chemical bonds are filled with H20, and the slow-growing facets fill up more slowly until the facet is complete!--this faceting process creates the hexagonal prisms that make up snow crystals.
Perhaps one of the most confusing aspects of snowflakes is how they form such intricate patterns, when all it really is is water vapor condensing into ice. When water molecules travel through the air towards a growing snow crystal, they don't just cruise--they must diffuse through the air. This diffusion slows their growth considerably, because the longer the distance they must diffuse, the longer it takes for them to reach the crystal. If an ice crystal is growing in the atmosphere, and a bump develops on the surface, then it attracts the diffusing water molecules because it is sticking out more than just the ice surface. The distance that they have to diffuse is less, so as more water molecules join onto the bump, the bump grows very quickly. In no time these small bumps develop into larger branches, which is called branching instability. The instability is a main factor for building the highly complex forms of snow crystals.
The overall shape of snow crystals depends solely on crystal faceting and snowflake branching. The faceting creates simple flat surfaces on which the branching grows from. Because the shape depends so much on temperature and humidity, there is a wide variety of different snow crystal forms. It's snow-riffic!!!
As I mentioned before, when water molecules are traveling through the atmosphere they are slowed due to diffusion. We should all know by now (courtesy of the fabulous Mr. Perrin) that the molecules in a gas, a liquid, and a solid are in constant motion. This is measured by kinetic energy, which is defined as the energy of motion that is being used. Substances are constantly trying to find a state of equilibrium, in which their molecules are concentrated evenly across the surface. If you were to watch the molecules in movement, it would just look like they were bouncing off of each other at random, but they are actually doing it for a reason.This process is called diffusion.
As I mentioned before, when water molecules are traveling through the atmosphere they are slowed due to diffusion. We should all now by now (courtesy of the fabulous Mr. Perrin) that the molecules in a gas, a liquid, and a solid are in constant motion. This is measured by kinetic energy, which is defined as the energy of motion that is being used. Substances are constantly trying to find a state of equilibrium, in which their molecules are concentrated evenly across the surface. If you were to watch the molecules in movement, it would just look like they were bouncing off of each other at random, but they are actually doing it for a reason (this is entropy, the degree of disorder in a system). This process is called diffusion.
The closer the molecules in a substance are packed, the harder it is for the diffusion to occur, because there is less space for the molecules to move about in. As a result of tighter packing, the molecules do not diffuse very quickly so the process takes longer. This explains why it takes longer for water molecules to travel through air when snowflake branching is taking place.
http://www.physics.cornell.edu
/sethna/teaching/StatPhys/GIF
/Diffusion.gif
http://members.aol.com/profchm/graham.html
http://www.its.caltech.edu/~atomic/snowcrystals/primer/primer.htm