Ice, water and steam are different phases of the one compound H20. We tend to think of the transitions between these forms as occurring strictly at the melting point and at the boiling point 100C. However the change can be delayed, especially when going directly from the vapour to the solid, and often needs help to nucleate the first tiny crystals. On this window the initial crystallization took place on the threads left by small spiders and particles of debris. The dense white regions on some of the remaining surfaces are masses of minute crystals growing at a much higher water vapour concentration; probably from the photographer's breath.