Re: How do Snowflakes Form?

The details of this are still rather poorly understood, as far as I can tell. The basic process goes something like the following. As you probably know, a cloud is just water vapor (steam, if you like), and is composed of very tiny water drops. Now, because the cloud is high in the sky, where the air is thin, these water drops get cold, below the freeaing point of water. But it turns out that absolutely pure water won't freeze by itself, even if you get below 32 degrees F, or 0 degrees C (though it will freeze on its own if you get it extremely cold). Instead, the water drops need to stick to something else, like a particle of dust, or smog, or even a bacterium. Only then does the water droplet freeze.

Once this has happened, other water droplets will stick to the newly made ice-crystal, and also freeze, making it larger. Depending on the conditions, different shaped crystals will grow. Generally, we see snowflakes having a six-sided symmetry. We can get a rough idea why this is by noting that in a water molecule (two hydrogen atoms and one oxygen atom stuck together), the angle between the two hydrogen atoms is about 120 degrees, which is also the angle between two sides of a regular hexagon (six-sided figure where all the sides are the same length). So in some rough sense, water molecules like to "fit together" in six-sided shapes, though this isn't always true.

The links below contain more information on snowflakes:

http://www.gi.alaska.edu/ScienceForum/ASF8/828.html
http://www.pa.msu.edu/~sciencet/ask_st/100897.html
http://tqjunior.advanced.org/3876/snowfacts.html
http://miavx1.acs.muohio.edu/~dragonfly/snow/icensnow.HTMLX