In fact the passenger windows aren’t made of glass at all but a double layer of acrylic. The space between the two layers helps reduce noise and is ventilated to prevent a build-up of moisture – which would spoil your view of the clouds. Aircrafts's windows are heated throughout the flight, for two reasons. First, to prevent the formation of ice, but more importantly to prevent glass becoming brittle. As anyone who has ever made the mistake of pouring hot water onto a frozen windscreen will know, glass can crack easily when it is cold!