Window frost, sometimes called 'fern frost' from its appearance, starts to form on the outer surface of glass when the surface cools below that of the surrounding air by radiating its heat out to an empty night sky.

Those of us senior enough to have experienced non centrally heated houses with single glazing will remember window frost on the inside of the window. The frost starts in the same manner when the glass surface falls below 0°C.

Water molecules from the air condense onto rough (at an atomic level) nuclei – scratches, pits, dust particles. Once a tiny ice crystal has formed it grows across the cold surface. Crystal growth is fastest at edges rather than on the large smooth planes of facets. We see large individual crystal shapes because the initial growth nuclei are far apart. It’s possible that the more polished and cleaner is your glass, the larger the shapes.

The growth is dendritic, treelike, along the hexagonal crystal directions to give the characteristic ribbed or multi-pointed appearance. The complex but overall symmetric shapes of snowflakes are similarly produced. But, unlike snowflakes, window frost has sweeping curves. Why?