What makes snow stick?
It is a truth universally acknowledged, that a person in the presence of a snowfall, must be in want of a fight. But what exactly gives snow the properties that enable such a satisfying splat? Why does some snow pack down into the perfect sphere, but other snow is too powdery to push together?
Snow is made up of small crystals of solid ice, liquid water, and air. Often, the difference between good and bad snowball snow is reported as how “wet” or “dry” the snow is - and this is correct, to an extent, but the reason behind the “wetness” of snow is actually down to its temperature. In fact, temperature plays a big role in whether or not compressing snow by hand will produce enough pressure to weld the ice crystals together.
While it might sound counterintuitive, at temperatures below the melting point of ice, there is usually some liquid water present on the surfaces of ice crystals. The higher the temperature, the thicker these “quasi-liquid layers” are, and the higher the water content of the snow. If the temperature is high enough, then some of the ice crystals will begin melting in earnest, and the water content of the snow can be as high as 10% - enough that if you squeeze the snow, you can actually wring out the water like a wet towel. But water content alone is not the answer to perfect snow. To really understand what makes snow stick, we have to zoom in to look at ice on a molecular level.
Ice is a crystalline material, and the molecules are ordered in a very specific and regular way. This highly ordered structure is a result of hydrogen bonds between adjacent hydrogen and oxygen atoms from neighbouring water molecules. At the surface of a crystal, this molecular order breaks down, and the molecules are arranged in a way that looks and behaves more like a liquid. This disordered layer is thought to reduce the energy of the surface. If the water molecules at the surface were arranged in a crystalline way, some of the molecules at the edge would have unbonded free ends. When the molecules become disordered, these free ends can hydrogen bond with each other, resulting in a lower energy interface.
Because of the strong hydrogen bonds that form between adjacent water molecules, and the quite open, hexagonal array that the molecules adopt, ice is actually less dense than liquid water. This is the reason why ice floats in water, and also leads to the phenomenon of regelation: the melting of ice to water under pressure, followed by the refreezing of that water to ice when the pressure is removed. Increasing the pressure pushes the molecules closer together, and because water is more dense than ice, this added pressure means that the melting temperature of ice decreases.
A good demonstration of regelation is to hang a weighted metal wire over a block of ice. The pressure of the weights on the wire causes the ice directly underneath it to melt. In this small pocket of water, the wire is pulled down by the weights, and the water flows upwards. When the water is above the wire, it is no longer under a higher pressure, so it refreezes. The wire travels through the block of ice, melting and refreezing, until it drops out of the bottom, leaving behind a still intact, solid block of ice.
Similarly, when making a snowball, the act of squeezing the snowball will lower the melting temperature of the ice crystals. If the temperature is right, then this small amount of pressure will be enough to melt a portion of the ice as you compact the snow. This liquid water will sit between solid crystals of ice, so that when the pressure is released again, the melted ice will refreeze, and effectively weld the ice crystals together. What results is snow that is mechanically stronger and denser, and therefore more likely to stay together in a ball when thrown.
But if the temperature is too low, the pressure of your hands just won’t be enough to stick it together. Similarly, if the temperature is too high, the snow will be so wet that it forms a very dense, slushy mixture that might be a little too hard to throw at someone safely - a snowball fight is about fun after all, not about causing actual injury!