Formation, properties and climatic effects of contrails

Condensation trails (contrails) are aircraft induced cirrus clouds, which may persist and grow to large cirrus cover in ice-supersaturated air, and may cause a warming of the atmosphere. This paper describes the formation, occurrence, properties and climatic effects of contrails. The global cover by lined-shaped contrails and the radiative impact of line-shaped contrails is smaller than that assessed in an international assessment in 1999. Contrails trigger contrail cirrus with far larger coverage than observed for line-shaped contrails, but still unknown radiative properties. Some model simulations indicate an impact of particles and particle precursors emitted from aircraft engines on cirrus clouds properties. However, the magnitude of this effect cannot yet be assessed. Contrail formation can be avoided only by flying in sufficiently warm and dry air. The formation of contrail cirrus can be reduced by avoiding flights in ice-supersaturated regions of the atmosphere, e.g., by raising the flight level into the lower-most stratosphere.