This looks like a spectacularly beautiful, but typical, example of surface hoarfrost.

There are many ways in which frost can form, and a correspondingly large number of forms that the frost can take, but what it looks like here is that there already was some frozen moisture on the surface, at several degrees below freezing point. What next happened was probably that a mild breeze carried some cold air, very likely also slightly below freezing point, but not as cold as that frozen surface, and with a moderate humidity, over the still colder, unadorned surface. Individual water molecules in the moving air, rather than condensing water droplets, then tended to land and stick most tenaciously to those surface spots where the charge and shape were such as to fit them best into the growing crystal. At less favourable spots water molecules were likely to evaporate again instead of remaining in the crystal.

The effect of this process is for the favoured spots on the crystals to grow, thereby creating spikes, flakes, and similarly feathery shapes. In the photograph it does not look as though the flakes have any particular orientation, so it is unlikely that the wind was very strong, at least not close to the surface.

You can put this observation down to one of the everyday treats that a little alertness can earn you. Keep your eyes open for more. You won't often find much, but when you do it is worth it!

Jon