Contrails (condensation trails) form in the exhaust of an aircraft engine. Water vapor is one of the by-products of combustion engines and when the exhaust reaches the cold upper atmosphere the water vapor condenses into water droplets and ice crystals. The contrail above has matured from an initial thin trail to this complex looking pattern. Here the contrail has spread into a broader cloud band that includes cirrocumulus and cirrostratus cloud types. The vertical streamers consist of falling ice crystals that form a narrow cloud band.
The Weather Briefing Blog