Snowflake Science!

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Example of a possible snowflake shape. Source: http://feelgrafix.com/group/snowflake.html

It honestly feels like Winter never really happened this year. I remember hearing rumours of snow on Christmas day sometime in November, and I must confess I got excited. Even though weather predictions are known to be near impossible if they’re after more than a few days.

But sadly that snow never happened, and in England at least we’ve had to settle with a near constant Autumn. Its true that we still have all of January and February for Winter to actually happen, but I think its unlikely that we’ll see any snow this year.

So since I still needed my snowy fix I decided to learn a bit about the formation of snow and how that knowledge can be of use to us. It may not be actual snow, but it might make your imaginary snow a bit more realistic.

Let’s start with what a snowflake actually is! Its a pretty broad term, with a huge variety of structures qualifying as a snowflake. The only concrete part of the definition is that the structure consist of more than one snow crystal, which is a single crystal of ice. These form in clouds when water vapour condenses into ice, a process which as two specific conditions for occurring.

The first is known as “Supersaturation”, which occurs when the amount of water vapour in the air exceeds the ordinary humidity limit. What does this mean? Well at every available temperature there is a maximum amount of water vapour that can be supported, with a higher temperature allowing for more water vapour.

If we cool a volume of air that’s already at 100% humidity then it now contains more water than is stable, and has become supersaturated. The excess water will then condense out, either into water droplets or directly into ice.

The second condition is “Supercooling”, or rather the lack there of. This is when a substance remains in a liquid state below its freezing point. It is possible for pure water to remain in a liquid state below 0oC, as the thermal motion of the molecules prevents crystallisation. In fact, the temperature has to drop to -42oC before freezing will occur!

On the other hand, tap water will readily freeze at 0oC due to the impurities it contains. These provide a surface for the molecules to cling to, reducing the effects of thermal motion. The scientific term for what these impurities provide is a “nucleation point”, a starting point for crystal growth. This also occurs in clouds when snow crystals form, as the many impurities such as dust and pollen particles provide nucleation points.

So! Now that we know what a snowflake is and the conditions for their formation, we can look at the process of crystal growth. It begins when the water molecules arrange themselves around the nucleation point. There are actually 14 possible lattice structures for ice, but ice 1h (short for “Form 1 Hexagonal”) is the most stable between 0 and 100oC, so its the most common form found in nature. In this arrangement the water molecules bond in a hexagonal lattice structure shown in Figure 1.

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Figure 1: The hexagonal lattice structure of an ice crystal. Red spheres represent Oxygen atoms, and white spheres represent Hydrogen. Source: http://www.thenakedscientists.com/HTML/articles/article/science-of-snowflakes/

The growth then continues as shown in Figure 2, with “rough” areas filling in faster than “smooth” ones. Why do they do this? Well a rough surface is one with multiple binding sites available, as more surface molecules are exposed. This makes it easier for incoming molecules to bind in these locations, and this growth pattern defines the hexagonal shape of the initial crystal.

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Figure 2: Diagram showing how additional water molecules bind as the ice crystal grows. Source: http://www.thenakedscientists.com/HTML/articles/article/science-of-snowflakes/

This crystal continues to grow as atmospheric water binds and becomes incorporated into the structure. However, from here on the growth is not uniform, with the corners growing fastest since they now offer the most exposed surface molecules. This is what causes the six “arms” that extend out from the corners of the central hexagon, and their size and shape will be determined by the ever changing conditions as the snowflake moves through the air.

The final shape of the snowflake will be determined by many factors including temperature, humidity, and how those conditions varied during it’s formation. This makes it extremely unlikely that you’ll ever find two identical snowflakes, as the number of possible combinations and variations is truly staggering.  Its actually made even less likely when you consider that the majority of snowflakes will not be perfectly symmetrical, as different parts of the snowflake can experience different conditions as well.

Now, while this is all very interesting, what is the actual point of studying the complex formation of snowflakes? Given that snowflake formation was successfully simulated by a research team from both Germany and London, it would be nice to know its not all for nothing! Well it turns out that this knowledge, while not having many immediate applications, could be very useful in the future.

Crystals are applied and used in many areas these days. These include semiconductor crystals for electronics, optical crystals for telecommunications, artificial diamonds for machining and grinding, the list goes on. So by studying snowflakes we gain a deeper understanding how crystals form and grow. Knowledge that may help us form new and better types of crystals in the future.

Some more interesting, and perhaps more important, things we can learn are the principles behind self-assembling structures. While us humans usually make things by carving structures out from a block of material, nature often has structures assembling themselves from smaller components. This production method will likely become HUGELY important as the electronics industry constantly moves towards smaller devices.

So now you can see that snowflakes are not only both beautiful and amazingly scientific, but also potential useful to us. Something I must confess I was unaware of before writing this post. Now while all this doesn’t change the fact that IT HASN’T SNOWED YET (this makes me very sad), at least you can madly rant about the science when it does.

Sources not mentioned in text:

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