Learning guide 1 · Atmospheric physics
How contrails form
A contrail is an ice cloud initiated by the mixing of hot, moist aircraft exhaust with very cold air. Formation takes seconds; what happens next depends mainly on the humidity of the surrounding atmosphere.
The short answer
Burning hydrocarbon fuel produces carbon dioxide and water vapor along with smaller quantities of other combustion products and particles. At typical cruise altitude, the surrounding air can be colder than −40 °C. As the exhaust plume expands and mixes with that air, it cools rapidly. Under the right temperature, pressure, humidity, and engine conditions, the mixture briefly reaches saturation with respect to liquid water. Tiny droplets form on particles and freeze, leaving a visible trail of ice crystals.
This explanation is testable. It predicts that the same aircraft can make a trail in one air mass and none in another; that different engines can have slightly different formation thresholds; and that a newly formed trail will vanish in dry air but grow in ice-supersaturated air.
Formation in five steps
Combustion adds heat and water
A jet engine burns fuel with air. The plume exits hotter and more humid than the surrounding upper-tropospheric air. Soot and background atmospheric particles can provide surfaces on which droplets form.
The plume expands
Turbulence entrains ambient air into the exhaust. Expansion and mixing lower the plume temperature while diluting its water vapor. The relevant state is the changing mixture, not exhaust or ambient air considered alone.
The mixture crosses water saturation
If the ambient air is cold enough, the mixing trajectory reaches saturation with respect to liquid water. This threshold is described by the Schmidt–Appleman criterion.
Droplets freeze
At cruise-level temperatures, the newly condensed droplets freeze rapidly. The visible line is therefore principally an ice cloud, not a trail of liquid droplets.
The environment decides its lifetime
Ice crystals sublimate in ice-subsaturated air. In ice-supersaturated air they can take up ambient water, grow, spread, and eventually resemble natural cirrus.
The Schmidt–Appleman criterion
Ernst Schmidt and Herbert Appleman developed the thermodynamic foundation; Ulrich Schumann later provided a widely used formulation incorporating propulsion efficiency. The criterion determines a threshold ambient temperature below which the exhaust–air mixing line can reach liquid-water saturation. Its inputs include ambient pressure and humidity plus engine/fuel properties such as water emission, combustion heat, and propulsion efficiency.
Formation is not the same as ambient cloud saturation. A short-lived contrail can form even when the surrounding air is not saturated because the exhaust adds water locally. Persistence, however, depends on the ambient air sustaining the ice after the plume dilutes.
A rigorous calculation needs more than surface weather and a flight altitude. It needs pressure-level temperature and humidity plus assumptions about the aircraft/engine. This is why Contrail Research labels its map output as experimental rather than claiming an exact engine-specific calculation.
What is in a visible contrail?
The visible portion is made of ice crystals that scatter sunlight. Engine exhaust supplies the initial water vapor and particles that help nucleation, while a persistent trail can accumulate much more water from the surrounding atmosphere. The joint EPA/FAA explanation emphasizes that persistent contrails are principally atmospheric ice cloud; visibility does not provide a chemical inventory of trace exhaust constituents.
| Ingredient or process | Role | Visible from the ground? |
|---|---|---|
| Water vapor | Produced by combustion and already present in ambient air; becomes ice after condensation and freezing. | The resulting ice cloud is visible, but vapor itself is not. |
| Ice crystals | Scatter sunlight and create the white, gray, orange, or red appearance. | Visible collectively, not as individual crystals. |
| Particles | Provide condensation/freezing nuclei; sources include engine exhaust and background air. | Ordinary images cannot determine particle chemistry or origin. |
| Wind and shear | Transport, stretch, and spread the trail after formation. | Inferred from motion over time, not from one frame. |
Why a trail can start, stop, cross, or curve
Broken or dashed lines
Upper-tropospheric humidity is patchy. An aircraft can pass through alternating regions that support a visible trail and regions where crystals evaporate quickly. From the ground, this can look like an abrupt on/off switch even though engine operation is continuous.
Crosses and grids
Airways intersect, traffic arrives from different directions, and trails at different times can drift into the same view. A two-dimensional crossing does not establish that aircraft met, flew at the same altitude, or created the lines simultaneously.
Curves, circles, and arcs
Aircraft follow turns, holding patterns, test patterns, and curved routes; winds can also deform older trails. The FAA documents both straight and circular contrail patterns. Shape is therefore useful context, not evidence of motive.
Color
White ice clouds can appear yellow, orange, red, gray, or iridescent depending on solar angle, particle size, optical depth, and the observer’s geometry. Color alone cannot identify composition.
What a visible line can and cannot establish
The best field record is a short time series, not one cropped image. Preserve the original file, UTC time, approximate location, compass direction, and several frames showing how the line changes. Then compare the observation with flight and weather data while remembering that neither dataset is complete.
Sources
- Schumann, U. (1996). On conditions for contrail formation from aircraft exhausts.
- Federal Aviation Administration: Contrails.
- U.S. EPA: Information on Contrails from Aircraft.
- NASA Langley: Contrails—formation and effects (technical educational monograph).