Contrail

Contrails or vapor trails are condensation trails and artificial cirrus clouds made by the exhaust of aircraft engines or wingtip vortices which precipitate a stream of tiny ice crystals in moist, frigid upper air. Being composed of water, the visible white streams are not, in and of themselves, air pollution. However, vapour trails or contrails generated by engine exhaust are inevitably linked with typical fuel combustion pollutants.

The main products of hydrocarbon fuel combustion are carbon dioxide and water vapor. At high altitudes this water vapor emerges into a cold environment, and the local increase in water vapor can push the water content of the air past saturation point. The vapor then condenses into tiny water droplets and/or deposits into ice. These millions of tiny water droplets and/or ice crystals form the vapour trail or contrails.
(courtesy: Wikipedia)

Was fortunate to capture this on a clear bue sky. When the trail is far away in the sky, can be mistaken for some kind of UFO stuff!