You might think that sounds like a dumb question: why does it matter how many molecules there are, they just need to be cold enough, right? Wrong: you need enough to make the complex crystal structure that you see so clearly in snow flakes -- and now scientists have worked out exactly how many that is.
275. But how did they get an answer so accurate?
Easy: when you shine light through water, it absorbs one particular frequency. Shine it through ice, and it absorbs a different one. Using some clever witchcraft -- OK, science -- a team of researchers from Germany were able to add water, molecule by molecule, into a vessel held well below zero. When they saw a change in the frequency of light the sample was absorbing, they had an answer.
Turns out that to form the beginning of any ice crystal requires 275 molecules of water. It isn't, however, a fully-formed crystal until 475 molecules are present. The result is published in Science.
What's crazy is that this is the first-ever experiment to explain what's happening as water molecules form ice crystals. I'll never look at a snow flake in the same way again. [Science via Scientific American]
Image by AMagill under Creative Commons license