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What atmospheric conditions affect contrail persistence?
Executive summary
Persistent contrails require ice-supersaturated air (relative humidity with respect to ice, RHi > 100%) so that contrail ice crystals do not sublimate and can grow; under those conditions contrails can last hours to days and evolve into contrail cirrus [1] [2] [3]. Predicting persistence depends primarily on upper‑tropospheric temperature and humidity structure (including lapse rate and mesoscale humidity variability), while engine exhaust properties and aircraft mixing also modulate formation and persistence [4] [5] [6].
1. The basic physical requirement: ice supersaturation controls persistence
Contrails form in the wake when the exhaust plume meets cold cruise‑level air and ice crystals nucleate; a contrail will persist only if it remains inside an ice‑supersaturated region (ISSR) where RHi exceeds 100%, allowing ambient water vapor to deposit onto ice and grow the crystals instead of causing sublimation [1] [3] [7].
2. Temperature and the Schmidt–Appleman criterion: when formation is possible
Immediate contrail formation (the young, visible trail) is governed by the Schmidt–Appleman criterion, a thermodynamic threshold that links ambient temperature, pressure and exhaust properties to whether plume cooling produces ice; colder, higher‑altitude air generally makes formation more likely [8] [9]. But formation alone does not guarantee persistence — persistence needs ISSRs [8] [3].
3. Humidity structure: the biggest practical forecasting barrier
Persistence is highly sensitive to upper tropospheric humidity. Forecast and analysis systems struggle because humidity at cruise altitudes has sharp spatial gradients and limited observations; accurate, high‑resolution temperature and humidity profiles are essential to predict ISSRs and thus persistent contrails [4] [5] [10].
4. Dynamics and stability: lapse rate, vertical motion and mesoscale context matter
Beyond RHi, dynamical variables such as local lapse rate (vertical temperature gradient), geopotential height, vorticity and vertical velocity correlate with whether contrails persist or dissipate. Studies suggest the lapse rate is among the best predictors to improve persistence forecasts, because it reflects stratification that controls mixing and crystal settling/growth [5].
5. Ambient clouds and “natural cirrus” interactions change the outcome
Recent work finds many long‑lived contrails form within or on top of existing cirrus rather than clear sky, meaning the local cloud field and its radiative/dynamical context influence contrail lifetime and climate effect; contrails inside natural ice clouds may merge with or modify those clouds, complicating simple persistence rules [11].
6. Aircraft factors: engine exhaust and plume mixing modify thresholds
Aircraft‑dependent properties — fuel and engine exhaust composition, and wake mixing (mixing line slope) — affect the Schmidt–Appleman threshold and initial plume microphysics. Models indicate design choices can alter the probability of persistent contrail formation, so both environment and aircraft matter [6] [9] [12].
7. Temporal and spatial variability — persistence ranges from minutes to days
Most contrails fade within minutes due to adiabatic heating and sublimation, but when formed in ISSRs they can last hours to days and spread into contrail cirrus that exerts measurable radiative forcing. This variability underlies why contrail climate impacts are uncertain and why mitigation (e.g., tactical rerouting) is challenging [3] [2] [4].
8. Modeling and observational limits drive large uncertainties
Global and regional models differ in predicted ISSR frequency and hence contrail persistence estimates; for example, host climate model differences change that frequency and thereby contrail effective radiative forcing estimates, producing substantial uncertainty (~tens of percent or more) [8]. Observational gaps in upper‑tropospheric humidity and temperature are repeatedly cited as the main limit to improving forecasts and assessments [4] [10].
9. Why this matters for mitigation and policy choices
Because persistence — not just formation — determines the climate impact of contrails, strategies to reduce aviation’s non‑CO2 warming require accurate ISSR forecasts, better humidity observations, and consideration of aircraft design and operational choices (e.g., altitude changes). Multiple reports and roadmaps call for enhanced data collection and coordinated research to reduce the uncertainty and enable targeted mitigation [2] [4] [10].
Limitations and open questions: available sources consistently link persistence to ice supersaturation, temperature and humidity structure and note aircraft factors, but they do not provide a single operational recipe for predicting individual contrail lifetimes; improving that capability depends on better upper‑tropospheric humidity observations and improved coupled models [5] [4].