Even when watching it with the naked eye, the smoke and fog of the fireworks seemed to show iridescent colours. The photographs (photo) show the iridescence more obviously. It was probably caused by the condensation nuclei from the fireworks smoke, on which small droplets condensated. As short time before an area of precipitation had passed, air humidity was still very high.
Additionally, the fireworks caused a thin layer of fog over almost the whole alpine foothills area (photo), and the big city of Munich with more than a million inhabitants, was covered by a thick layer of clouds (photo).
Similar things were reported by other observers. In Bochum, Peter Krämer observed that light graupel turned into snow during the fireworks, leaving about a centimetre of snow. On the weather radar it could be seen that a precipitation area formed right over the Ruhr area just after midnight.
Two years ago, thin fog with visibilities around 300 meters thickended after the New Years Fireworks, forming a dense layer of fog with visibilities which were less than 10 meters in some places.