What Are Contrails?

Contrails, short for condensation trails and also called vapor trails, are line-shaped ice clouds that form behind aircraft when hot, humid engine exhaust mixes with cold ambient air at cruise altitude. Water vapor produced by the combustion of jet fuel, together with soot and other exhaust particles that act as condensation nuclei, briefly supersaturates the mixing plume with respect to liquid water. Droplets nucleate on those particles and freeze within seconds, leaving a visible trail of ice crystals typically at altitudes between 8 and 12 km.

Contrails sit at the intersection of atmospheric physics, combustion engineering, and air traffic management. They are studied largely because of their radiative effect: aviation's warming influence on climate comes not from carbon dioxide alone, and current assessments place the contribution of contrail cirrus at a magnitude comparable to that of aviation's accumulated CO2 emissions, though with considerably greater uncertainty.

Formation Conditions

Whether a contrail forms at all is governed by the Schmidt-Appleman criterion, a thermodynamic condition set out by Ernst Schmidt in 1941 and put into forecasting form by Herbert Appleman in 1953, which compares the mixing line of exhaust and ambient air against liquid water saturation. Formation depends on ambient temperature and humidity, on ambient pressure, and on the overall propulsive efficiency of the engine, since an engine that converts more of the fuel's energy into thrust leaves a cooler, relatively wetter exhaust plume. This is why higher bypass ratio engines can form contrails under conditions where older designs would not. Soot number concentration sets the initial crystal count, so fuel composition and combustor design influence the microphysical properties of the trail from the first moments of its existence.

Persistence and Contrail Cirrus

Most contrails dissipate within minutes. The consequential ones are those that form in air that is supersaturated with respect to ice, where the crystals do not sublimate but instead grow by taking up ambient water vapor. Such contrails spread under wind shear and turbulence into diffuse sheets that can persist for hours, cover thousands of square kilometers, and become visually indistinguishable from natural cirrus. Simulations described in Nature Communications on the formation and radiative forcing of contrail cirrus trace this life cycle from the vortex phase behind the wing through to the spreading stage that dominates the total climate effect.

Radiative Forcing and Mitigation

Contrail cirrus both reflects incoming solar radiation, a cooling effect, and absorbs outgoing longwave radiation from the surface, a warming effect. For thin, high, cold ice clouds the longwave term generally dominates, so the net effect over a full day is warming, with the balance shifting toward cooling for daytime contrails over bright surfaces. A review in Atmospheric Chemistry and Physics assesses the global magnitude of this forcing and the sources of remaining uncertainty, including ice crystal habit, overlap with natural cirrus, and the representation of the effect in climate models. Because persistent contrails are produced by a small minority of flights passing through ice-supersaturated regions, mitigation research focuses on forecasting those regions and rerouting the affected flights vertically, a trade against the extra fuel burned. Airborne and satellite observation campaigns coordinated through NASA research on contrails and cirrus trends supply the measurements used to test both the forecasts and the models.

Applications

Contrail research has applications in a range of fields, including:

• Climate modeling and radiative forcing assessment
• Satellite and ground-based remote sensing of cirrus
• Air traffic management and flight trajectory optimization
• Sustainable aviation fuel and combustor development
• Aviation environmental policy and emissions accounting
• Numerical weather prediction of upper tropospheric humidity