Yesterday morning, much of the UK woke up to snow. It was all very exciting, and much fun was had by all; the cats hated it; we built a comely snow wench. But where does snow come from? Why don’t we get more of it when it’s cold? And is it true that no two snowflakes are identical?
When we talk about snowflakes, sometimes we mean the pretty individual crystals of snow with their symmetrical patterns, and sometimes we mean the white things that fall out of the sky. Snowflakes are more accurately only the latter of these; usually clumps of multiple snow crystals. Snow crystals are the individual crystals themselves; single crystals of ice with all the water molecules arranged very precisely in a shape something like a hexagon.
Snow is not frozen rain. Frozen rain is sleet or hail. They form differently. Sleet and hail form when water vapour condenses to raindrops in the clouds and begins falling, and then freezes. Snow, on the other hand, only forms under very particular conditions, when the temperature and humidity (the amount of water vapour in the air; high humidity = lots of water vapour in the air) are just right, and you get water vapour turning straight to solid ice without becoming liquid water first. The initial crystal of snow usually forms on a speck of dust or pollen in the atmosphere. As more water vapour freezes onto it, it gets bigger and the intricate patterns form, starting from the corners of the initial hexagonal crystal. These exact conditions – without any air warm enough to melt the snow between the clouds and the ground – are actually quite uncommon over most of the UK, so we don’t see snow that often. They are also the cause of the different forms of snow – the stuff like we had over the weekend which is excellent for snowball fights and building snowmen, and the fine powdery stuff that doesn’t stick together and is somehow less ‘wet’.
When the temperature is just below freezing and the humidity is fairly low, ice crystals will form in simple hexagonal plates. As the amount of water vapour is increased (as the crystal falls through clouds, for example), dendrites (tree-like branches) start to form from the points where it is easiest for them to attach: the corners. These grow and grown as more and more water vapour freezes on to the crystal, giving the characteristic ‘fern-like’ snowflake shape we are familiar with. The exact form these dendrites take – and whether additional plates form on them – depends on the specific changes in temperature and humidity that a snow crystal experiences as it moves through the atmosphere. In many ways, each snow crystal is a map of its own history, and the path it has taken since its formation. This is why no two snow crystals are the same, because each crystal will take an ever so slightly different random path and so will form ever so slightly differently.
The symmetry of snow crystals is an interesting one. Each of the six corners of the snow crystal will experience the same conditions as it falls, and so will grow in the same way, leading to the six-fold symmetry we see. But within each crystal, the growth is random, and so we end up with this sort of ordered chaos. It’s a bit like a kaleidoscope – the completely random arrangement of beads/similar at the end of the kaleidoscope, reflected through the mirrors in the tube, gives rise to a regular pattern.
The first person to observe the hexagonal nature of snowflakes (or at least, the first one to document it) was Johannes Kepler – the very same one who did a lot of work on space. He was so intrigued by snowflakes that he wrote a book about them in 1611: The Six-Cornered Snowflake. In it, he suggested that the hexagonal nature of snowflakes might come from the close packing together of lots of identical units, a bit like honeycomb. He continued on to say that ice is made from tiny hexagonal patterns, and snowflakes are crystals of ice. What’s incredible is that this theory – which we now know to be pretty accurate – pre-dates atomic theory. At this point in time, the idea of matter being made of minuscule particles (‘atoms’) was just speculation by ancient Greek philosophers!
Snow is lots of fun, but you can do more with it than just build snowmen and throw snowballs at your siblings and friends. If you catch individual flakes, you can look at them with a magnifying glass and identify the shape – you might even be able to work out what sort of temperature they are formed at (there are plenty of charts on the internet to help you do this). See if you can spot the hexagonal plate in the middle. The Smithsonian Museum in America also has some good resources about how scientists started investigating snowflakes. A little more preparation, and you can actually preserve one – instructions are available here (as well as some liquid superglue, you’ll need a microscope slide and a cover glass – both easily obtained via the internet).