What scientific evidence explains why some contrails persist and spread into cirrus clouds?
Executive summary
Persistent contrails form when cold, moist upper-troposphere air mixes with hot, humid aircraft exhaust and the conditions meet a thermodynamic formation threshold; in regions that are ice‑supersaturated, those ice crystals can survive, grow and spread into contrail cirrus that resemble natural cirrus clouds [1] [2] [3]. Observations, modeling and radiative‑forcing analyses consistently show contrail cirrus exert a net warming effect on climate, but the magnitude and exact mechanics—especially persistence, spreading, and interactions with pre‑existing cirrus—remain areas of active research and significant uncertainty [4] [5] [6].
1. What a contrail is and the basic physics that creates one
A contrail begins when water vapor in jet exhaust encounters very cold ambient air at cruise altitudes and condenses onto combustion particles (soot and sulphate), forming liquid droplets that rapidly freeze into ice crystals that make the visible streak behind an aircraft [1] [3]. Whether a visible contrail forms at all is governed by the Schmidt–Appleman criterion—a thermodynamic test that determines if exhaust‑air mixing reaches saturation with respect to liquid water or ice—so formation depends on engine emissions, ambient temperature and pressure [2] [3].
2. Why some contrails vanish quickly while others persist for hours
Persistence is controlled not by the aircraft alone but primarily by the ambient humidity and dynamical state: if the flight path crosses an ice‑supersaturated region (ISSR), contrail ice crystals can avoid rapid sublimation and instead survive for hours, whereas in drier air the crystals sublimate within minutes [7] [8]. Studies show that predicting persistence requires not only the Schmidt–Appleman formation test but also precise knowledge of local ice supersaturation and turbulence—variables that are still imperfectly represented in weather and climate models [2] [8].
3. How persistent contrails spread and become contrail cirrus
Once contrail ice crystals survive, ambient wind shear, turbulence and microphysical processes cause the narrow trail to disperse and mix with surrounding air, increasing particle number and cloud fraction until the feature resembles natural cirrus—so‑called contrail cirrus [9] [5]. Large‑eddy simulations and parcel/microphysics models show that this lifecycle—from inception in the engine wake to spread and eventual decay—depends on crystal growth, aggregation, and interactions with background cirrus, with in‑cloud formation pathways commonly observed [9] [6].
4. The scientific evidence linking contrail cirrus to warming and the scale of the effect
Multiple syntheses and model‑based radiative‑forcing studies conclude that aviation‑induced cloudiness produces a net positive (warming) effective radiative forcing, with contrail cirrus a dominant non‑CO2 contributor to aviation’s climate impact [4] [5] [6]. Quantitative estimates vary—papers and reports indicate large uncertainties—but recent work attributes a substantial fraction of aviation’s non‑CO2 forcing to contrail cirrus and notes that a small share of flights may cause a disproportionate share of warming [4] [10] [6].
5. How well persistence and high‑impact contrails can be predicted and mitigated
Thermodynamic formation points can be forecasted with reasonable skill, yet predicting which contrails will persist remains challenging because numerical weather prediction models still represent ice supersaturation and small‑scale processes crudely; improved humidity observations, airborne sensors and model development are repeatedly recommended [2] [8] [11]. Operational mitigation trials—rerouting horizontally or vertically to avoid ISSRs and using improved forecasts—have shown promise and low fuel‑cost tradeoffs in some studies, but implementing contrail avoidance at scale raises network, safety and cost questions [12] [11] [13].
6. Where the uncertainties, competing interests and research gaps lie
Key uncertainties include sparse observational data that make contrail cirrus hard to distinguish from natural cirrus, imperfect model physics for ice supersaturation and turbulence, and sensitivity to aircraft design and fuel composition—issues that industry groups stress to argue for technological and operational fixes while independent researchers emphasize atmospheric observation and modelling gaps [5] [14] [4]. Sources from aviation stakeholders push mitigation feasibility narratives and cite small fuel penalties for rerouting, which can underplay integration challenges and residual scientific uncertainty that independent assessments still flag [13] [8].