Believe it or not, making ice is more complicated than just making water really cold. One thing that helps is bacteria. Yes, bacteria! In this captivatingly magic video, it takes just a second for Pseudomonas syringae to turn a whole jar of water into ice.
How does it work? It's the same principle behind how snow forms in the atmosphere (and in artificial snow machines, too—we'll get to that later). An ice crystal needs to form around a nucleus, which can be a bit of dust, soot, pollen, or, as we've seen, bacteria. Pure water doesn't have to crystallise into ice until it's as cold as -48 C below zero. In the demo here, the water has been supercooled to about -29 C, but it only freezes over after the P. syringae is added.
Maggie Koerth-Baker, who first spotted the video for Boing Boing, explains where P. syringae's cold superpower comes from.
P. syringae gets this skill from the proteins that cover its surface membrane. The proteins basically form a physical structure that water molecules latch onto. That structure also orients the molecules in a way that prompts the formation of ice crystals. It's these proteins that really serve as the instigator of ice nucleation and they're incredibly efficient at it — far more so than dust...
Commercial snow machines use the proteins (though not the bacteria itself) to help instigate the creation of snow on ski mountains.
In its earthbound life, P. syringae causes disease in plants, so its snow-forming ability might seem bizarre. One hypothesis is that ice nucleation is really a subtle means of hacking the weather system: bacteria drifts up into the atmosphere and falls back to earth as snow and hail, travelling hundreds of miles in between.