Condensation trails, commonly known as contrails, are line-shaped clouds composed of ice crystals frequently observed following jet aircraft across the sky. These clouds are essentially an artificial form of high-altitude cirrus cloud. The formation of these visible lines is the direct result of aircraft engine exhaust interacting with the surrounding atmosphere at cruising altitudes. Understanding why some jets produce these streaks while others do not requires looking closely at the physics governing how hot exhaust mixes with extremely cold, thin air.
The Thermodynamics of Contrail Formation
Fuel combustion in the jet engine produces extremely hot exhaust gases containing a large amount of water vapor. Jet fuel, which is primarily hydrocarbon, releases water and carbon dioxide upon burning, with water vapor being the most relevant component for contrail creation. This hot, moist exhaust is immediately expelled into the ambient air, which, at typical cruising altitudes, is far below freezing.
The rapid mixing and cooling of the exhaust plume cause a steep and sudden drop in temperature. This thermodynamic change forces the water vapor content of the exhaust to quickly exceed the saturation limit of the surrounding air, leading to condensation. The water vapor condenses into microscopic liquid droplets, which then instantly freeze into ice crystals because of the frigid environment, forming the visible components of the condensation trail.
Crucially, the exhaust also contains microscopic particles, such as soot or unburned fuel components, which act as condensation nuclei for the water vapor. Without these tiny solid particles, the transition from vapor to ice would be much less efficient. The initial formation is governed by the Schmidt-Appleman criterion, which defines the necessary balance between the heat and moisture added by the engine and the pressure and temperature of the ambient air.
The Critical Role of Atmospheric Conditions
The intermittent nature of contrail sightings—where one jet leaves a trail and another nearby does not—is entirely dependent on external atmospheric conditions. For a contrail to form at all, the ambient air must be sufficiently cold, typically falling below a threshold of approximately -40°C to -41.1°C. However, cold alone is not enough; the air must also possess a high ambient relative humidity with respect to ice.
The specific atmospheric zones where trails form and persist are called ice-supersaturated regions (ISSRs). In these regions, the air contains more water vapor than is necessary to be in equilibrium with ice, meaning the relative humidity with respect to ice (\(RH_I\)) is above 100%. If the air is cold but not sufficiently humid, the water vapor condensed from the exhaust will re-evaporate almost instantly into the surrounding drier air, resulting in no visible trail or one that disappears within seconds.
These necessary temperature and humidity levels are found within the upper troposphere, where long-haul commercial aircraft typically cruise. Even small variations in altitude or flight path can move an aircraft into or out of an ISSR, which explains why a jet may abruptly start or stop producing a trail.
Why Contrails Persist or Dissipate Quickly
Once a contrail has formed, its visual appearance and lifespan are determined by the subsequent interaction of the ice crystals with the atmospheric moisture. Contrails that form in air that is cold but not ice-supersaturated are known as non-persistent contrails. In these conditions, the newly formed ice crystals rapidly transition directly from a solid state back into water vapor, a process called sublimation, causing the trail to dissipate quickly.
In contrast, persistent contrails form when the air is highly ice-supersaturated, providing a continuous supply of water vapor for the ice crystals to grow larger. These persistent trails can remain visible for hours, allowing them to widen and spread out as they are acted upon by air currents. The initial, narrow exhaust plumes can grow into wide, diffuse sheets of cloud, often becoming meteorologically classified as Cirrus homogenitus, a type of cirrus cloud.
The shape and evolution of a persistent trail are significantly influenced by atmospheric dynamics like wind shear and turbulence. Wind shear, the difference in wind speed or direction over a short distance, can stretch and distort the contrail, spreading the ice crystals over a much larger area. Initially, the wake vortex from the aircraft wings also influences the shape. Eventually, the trail enters the “dispersion regime” where it blends into the general high-altitude cloud cover, sometimes becoming indistinguishable from natural cirrus.