Formation of a dissipation trail (Distrail) on the photographed Cirrus vertebratus (Ci vert) cloud due to a flying aeroplane. Distrails such as these are formed in 2 ways: (1) Either because of evaporation due to the heat emitted by the plane`s exhaust or (2) Due to the freezing of super-cooled water droplets by adding nuclei on which water droplets can freeze. It is difficult to tell which process took place from just a photo. As proved in the second thumbnail photo, contrails and distrails can occur simultaneously. The weather sounding (first thumbnail) clearly showed a very moist airmass at an altitude of between 8.5km and 12km practically up to jet stream level. Such condition is always a must for contrails (and indeed distrails) to form. Ideally lower level airmass should not be too humid as then lower-level clouds tend to hide these features. Contrails are also known as Cirrus aviatus. The fourth thumbnail shows a photo of cirrus spissatus cloud taken on the same day which was too thick to produce irridesence or indeed solar halos. An upper-level trough was approaching the area.