Why Do Planes Leave Contrails?
Water vapor + ice crystals at altitude · Updated May 2026
Contrails ("condensation trails") are line-shaped clouds made of ice crystals. They form when hot, moist exhaust from jet engines mixes with the very cold, low-pressure air at cruising altitude (typically 26,000–40,000 ft, where the ambient temperature is below about −40 °C). Water vapor in the exhaust condenses onto microscopic soot and sulfate particles, then freezes within a fraction of a second into ice crystals — the white line you see behind a high-altitude jet. Whether the contrail is short-lived (a brief streak) or persistent (a long, spreading band) depends on the humidity of the air the aircraft is flying through.
The physics in one paragraph
A jet engine burns kerosene with atmospheric oxygen. One of the combustion products is water vapor — roughly 1.25 kg of water for every kilogram of fuel burned. That vapor exits the engine at a few hundred degrees Celsius. Within tenths of a second it mixes with ambient air that may be at −50 °C or colder. The mixture becomes briefly supersaturated with respect to water; tiny droplets condense on soot and sulfate particles from the exhaust; those droplets then freeze almost immediately, because the surrounding air is well below freezing. The result is a line of ice crystals trailing behind the aircraft.
The threshold conditions are described by the Schmidt–Appleman criterion, a standard physics relationship used in atmospheric science to predict when a given engine, at a given altitude, fuel type, and humidity, will produce a contrail.
Short-lived vs persistent contrails
Not every contrail looks the same — and the difference matters for climate science.
Form in dry upper-air layers. The ice crystals sublimate (evaporate directly back to vapor) within seconds to a minute. You see a brief streak that fades as the aircraft moves on. Climate effect: negligible.
Form in air that is ice-supersaturated — it already holds more water vapor than equilibrium would predict for ice. The contrail's ice crystals don't sublimate; instead, they grow by pulling more water vapor out of the air. The line broadens, spreads in the wind, and can persist for hours, sometimes evolving into a thin "contrail cirrus" cloud sheet covering large areas.
If the ambient air is too warm or too dry, the Schmidt–Appleman criterion isn't met and no contrail forms. Lower-altitude jets, hot summer flight levels over deserts, and some equatorial regions often see contrail-free traffic.
Why contrails are a climate topic
Persistent contrails behave optically like thin cirrus clouds. During the day, they reflect some incoming sunlight back to space (a small cooling effect). But — and this is the key finding — at night and over the full 24-hour cycle, they trap outgoing infrared radiation more than they reflect sunlight, producing a net warming effect on the climate.
According to a 2025 National Academies of Sciences report, aviation's non-CO₂ effects — contrails, nitrogen oxides, and aerosols combined — may contribute roughly as much warming as aviation's direct CO₂ emissions. That has made contrail avoidance an active area of research: rerouting a small percentage of flights around ice-supersaturated regions could meaningfully reduce aviation's climate footprint without large fuel penalties.
NASA, NOAA, and academic groups (including ongoing work at NASA Earthdata, NOAA NESDIS, and atmospheric science journals like ACP) are mapping ice-supersaturated regions using satellite humidity data and model forecasts, with the goal of giving flight planners contrail forecasts alongside winds and turbulence.
Common myths about contrails
- ×"They're chemicals being sprayed." No. Contrails are water-ice clouds. Their composition has been measured in situ by atmospheric research aircraft for decades. The visible material is frozen water — the same substance as natural cirrus clouds.
- ×"Long contrails are different from short ones." Same physics, different ambient humidity. The long, spreading ones simply happen to be flying through ice-supersaturated air.
- ×"Only some aircraft make contrails." Any jet engine in the right atmospheric conditions will form one. The same flight on a different day may or may not leave a visible trail.
What about hydrogen and sustainable aviation fuel?
Sustainable aviation fuel (SAF) typically contains fewer aromatic compounds, which reduces soot emissions. Fewer soot particles means fewer ice nucleation sites in the exhaust, which means smaller ice crystals — and shorter-lived, less radiatively significant contrails. Flight testing programs over the past several years have measured meaningful contrail reductions with SAF blends.
Hydrogen combustion is more complicated. It produces 2.6× more water vapor per unit of energy than kerosene and emits no soot. Recent modelling work (ACP, 2025–2026) suggests hydrogen contrails would be fewer in number but, when they form, potentially as climate-relevant as today's — an active research question.
What this means for passengers
- →Seeing a contrail behind your aircraft (or any other one) is normal physics, not an anomaly.
- →Contrails have no effect on flight safety. They are a downstream byproduct of normal engine operation.
- →The climate impact, however, is real and is now part of how the aviation industry measures its environmental footprint. Expect more public discussion of "contrail avoidance" routing in the coming years.
Sources
- • NASA Earthdata — "On the Trail of Contrails" overview of satellite-based contrail observation.
- • NOAA NESDIS — Contrail Simulation educational resource on formation conditions.
- • Atmospheric Chemistry and Physics (ACP) — Peer-reviewed work on contrail microphysics, including hydrogen-aircraft modelling (2025, 2026).
- • National Academies of Sciences — 2025 report on aviation non-CO₂ climate effects.
- • AIAA Aerospace America — "A closer look at contrails."