Air traffic may increase cirrus cloudiness

High-level cirrus clouds can evolve1, 2 from the condensation trails of aircraft, which form as the mixture of warm, humid exhaust gases and colder, drier air exceeds water saturation3. In addition, the particles in exhaust plumes from aircraft may allow ice nucleation at lower supersaturations than those required under natural conditions4. This mechanism is sensitive to environmental conditions, but may occur downstream of the exhaust aerosol source regions. Here I show that cirrus clouds increased in occurrence and coverage in the main air-traffic flight corridors between 1982 and 1991.