“THERE ARE CLEAR AND FROSTY MORNINGS, WHEN THE OUTSIDE WORLD HAS TRANSFORMED ITSELF INTO A TWINKLING DISNEY SET”
The weather at the moment is a bit grim: grey and cold with a generous dash of sleet. But there are clear and frosty mornings that make up for it, when the outside world has transformed itself into a twinkling Disney set. Yet when you look at the frost, it isn’t evenly distributed. Plants and wooden benches are often covered in it, but metal railings and the patches of ground beneath trees aren’t. It looks as though frost is an indicator for something, but what is it?
The beauty of frost comes from the moment of its formation. When we think of water cooling, we tend to assume that water vapour will condense to form liquid water, which will then freeze to form ice. But frost is assembled from thin air and bypasses the middle stage completely. Molecules of water in the air bump into an ice crystal and just freeze directly onto it, dropping into place on the existing crystal structure. It turns out that a floating water molecule is more likely to join the frozen crowd if there’s a space for it on a rough surface, so lumps and bumps get filled in and nice, smooth crystalline facets form. But that doesn’t answer the question about where frost is most likely to happen.
For frost to form, a solid surface needs to be below 0°C, and there needs to be sufficient water in the air. When it’s close to freezing, the air is generally very dry already, but if there are more than five water molecules in every thousand air molecules, the air is officially supersaturated, and frost can be water’s route out. This is part of the puzzle – pockets of cold, humid air are where you will find frost. For example, some dark surfaces radiate their heat away quickly, and so they will cool more than their surroundings, dropping below the temperature needed for frost. This explains why there’s often a frost-free patch under a tree. The tree is insulating the ground around it, preventing the soil from dropping below the critical temperature.
But there’s one extra condition for frost formation. Even if the temperature and humidity are right, frost may still not form. And that’s because, for that first floating water molecule to freeze when it bumps into a solid surface, that surface has to have the right structure for it to lock on to. If there’s already ice there, that’s perfect – the new molecule can just slot into its place in the ice crystal. If not, you need a nucleus. This is a starting point that provides the right structure, like a flat Lego baseplate that lets you position the first bricks. Plants are often beautifully frosted because bacteria on their surface play the nucleus role. There are a few types of bacteria that do this, and they’re extremely common. Without this coating of biological ice nuclei, plants would stay frost-free to lower temperatures. But nature is full of bacteria, and frosted lawns and hedges are the result.
So that cheerful frosty morning can be appreciated on two levels. There’s the white sparkle itself, and there are the invisible patterns of temperature, humidity and ice nuclei that are revealed in the twinkling. But frost appreciation is a chilly hobby, and the sort of thing that earns you a hot drink when you get back inside. Brrr!