Science In Real Life

Welcome to the first installment of Science In Real Life. Today’s subject is snow, something we’re all familiar with, except you weirdos in obscenely temperate climates or gratuitously low latitudes. Snow is a lot more complicated than it first appears, and the long-haired term for the area of physics I’ll be discussing is thermodynamically-governed crystal formation.

I know what you’re thinking – Water gets cold, it freezes solid and then falls to the ground. What’s to get? And that’s more or less the nutshell version of it, if you’re into small and really boring nutshells. It’s like saying “The Beatles made some cool music”. We need more information.

Some basics first. There’s quite a bit of water in the atmosphere, and as the temperature drops the molecules of H2O go from a gaseous state (floating around without any attachments to other molecules) to a liquid state, where they all clump together in a droplet around a piece of dust like pre-teen girls around a copy of Twilight. From there, if it gets colder still, the droplet will freeze into an ice crystal, also known as a snowflake.

But that’s where the complications set in. The precise shape that the ice crystal takes is surprisingly intricate, and it is the end result of several very variable factors, with the end result of a high degree of diversity among the population of snowflakes. You may have heard that no two snowflakes are identical – well, that’s not completely true, but it’s close enough for government work.

The primary guiding principle under which this crystal forms is the trademark six-sided symmetry (okay, so no one has registered snowflakes with the US Patent Office just yet, but give it time). This arises because of the Mickey-Mouse’s-head shape of the water molecule. The oxygen atom, like a schoolyard bully, beats up the hydrogen atoms and takes their lunch money, or in this case their electrons. Since the electrons are on one side, and the positively charged hydrogen nuclei are on another, the water molecules can only stick to each other at certain angles. Specifically, opposite charges need to be matched up.

Think of it like a jigsaw puzzle, or a tesselation, only in three dimensions. What really creates the hexagonal pattern is the angle between the two hydrogen atoms (104 degrees and change).

(Image credit)

This picture, if you stare at it long enough, should give you an idea of how the general rules for water molecules sticking together gives rise to hexagonal formations. If you’re having trouble seeing it, try to imagine looking at it from a different angle. The lines drawn between the molecules represent the chemical bonds, and they very handily also form the sides of a regular polygonal arrangement.

Now, that hexagonal structure is what happens in every instance of liquid water becoming solid ice. So what about snowflakes? Well, consider an ice cube, several centimeters on a side. It’s far too large to exhibit any signs of its internal structure. But a snowflake is only a few millimeters in diameter, so like a small child it will happily show you exactly what it’s doing (over and over and over again). Put another way, and with less recourse to making fun of children, your ice cube over there is made up of hexagonal structures, but it has so many of them put together you can’t see the very-small-scale hexagonal edges.

Let’s talk about diversity, and I don’t mean affirmative action. What causes all those variations in snowflake shapes? The formation of snowflakes is a very hectic process. Imagine you’re five years old again, sitting in kindergarten. The entire class is sitting at their desks, in a very orderly pattern. Then the teacher has you all stand up, and says “Okay, everyone form groups!” Bang! You’re off like a shot, trying to grab onto the nearest three or four kids. But since everyone is going in all directions, some groups end up with two kids and some end up with a whopping six or seven, and all the groups have kids standing in different arrangements.

That’s what the formation of snowflakes is like. It follows general rules about how the kids, or water molecules, stick together, but diversity of shape is caused by local variations of the following factors:

1) Temperature (in this case, how fast the molecules are travelling)
2) Pressure (how much the water molecules are pushing on each other)
3) Humidity (how many water molecules per given volume of air)

All of this can lead to radically different snowflake shapes than ones most of us are used to. See this diagram:

(Image credit)

All of these shapes are possible because they are all governed by the rule of hexagonal symmetry, and they will form under the different conditions as indicated by the graph (supersaturation in this case is like a combination of pressure and humidity, in case you were wondering).

So, there you have it, ladies and gentlemen. Everything you never knew about snow. Oh, one last thing: it’s cold.