|A reminder from the Southern Hemisphere (if one be needed!) that winter is now officially with us north of the equator and that we have the magic of frost. Shot in July in Dunedin, New Zealand by Steve Kerr, this frost was on the outside surface of his car windscreen. He used his iPhone to image it from the car inside. ęSteve Kerr, shown with permission.|
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.
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.