From Earth Science Picture Of The Day; July 26, 2014:
Crystals and Droplets on Airplane Windows
Photographer: Bertrand Kulik; Summary Authors: Bertrand Kulik, Jim Foster
The photos above showing tiny water droplets and much larger ice crystals were taken from the window seat of a jet airliner. At the altitudes jet aircraft fly, the air temperature may be -40 F (-40 C) or even lower. Water vapor crystallizes at these temperatures if condensation nuclei (dust and other& aerosols) are present. However, it’s likely that these droplets and crystals are in between panes of glass, where the temperature and pressure supports both droplets and ice. The odd shape of the crystals results from scratches and other flaws in the window glass – the crystals grow about these imperfections. Note that the area around the crystals is free of droplets since the vapor there was drawn to the crystals – vapor typically moves from warmer to colder surfaces.