When looking up at the sky, one of the most common sights is a jet aircraft tracing a bright white line across the blue expanse. These streaks are a meteorological phenomenon known as contrails, short for “condensation trails.” Contrails are artificial clouds created by the interaction between the hot exhaust from jet engines and the frigid air of the upper atmosphere. Understanding these trails requires examining the atmospheric conditions and physics that allow them to form and sometimes linger.
The Physics of Contrail Formation
Contrail formation results from the thermodynamics involved when two air masses of different temperatures and humidity levels mix. Jet engines burn fuel, producing a hot exhaust plume consisting mainly of carbon dioxide and a large amount of water vapor. This water vapor is a byproduct of combustion.
The aircraft’s cruising altitude, typically between 25,000 and 40,000 feet, has extremely low ambient air temperatures, often below \(-40\) degrees Fahrenheit. As the moist exhaust plume exits the engine nozzle, it instantly mixes with this surrounding cold air. This rapid mixing causes the exhaust gas temperature to drop precipitously, leading to a state of supersaturation.
Supersaturation occurs when the air contains more water vapor than it can hold in a gaseous state at that temperature and pressure. The excess water vapor condenses into microscopic liquid water droplets, which immediately freeze into tiny ice crystals due to the intense cold. The exhaust also contains minute soot particles that act as nuclei, providing surfaces for the water vapor to freeze onto and accelerating the formation of the visible ice cloud.
This process is similar to seeing your own breath on a cold winter day, where warm, moist air condenses upon hitting the frigid outside air. The combination of the engine’s water output and the low ambient temperature must meet a specific threshold, known as the Schmidt-Appleman criterion, for a contrail to form. If the air is not cold enough, or if the exhaust’s heat dissipates too slowly, the contrail will not appear.
Why Trails Persist or Dissipate Quickly
The duration and appearance of a contrail are governed by the atmospheric conditions, particularly the relative humidity of the air mass. If the surrounding air at cruising altitude is relatively dry, the ice crystals quickly sublimate, turning directly from a solid into an invisible gas. These are called short-lived contrails and disappear within seconds or minutes of being created.
When the air mass is moist or near saturation, the contrail enters a stable environment where the ice crystals do not readily evaporate. These are known as persistent contrails, and they can remain visible for hours, sometimes even longer than a day. The high relative humidity allows the contrail to sustain itself and even grow as it attracts more water vapor from the surrounding atmosphere.
Persistent contrails often evolve into a third type: persistent spreading contrails. As the initial narrow line of ice crystals is acted upon by upper-level winds and wind shear, it begins to diffuse and widen. This spreading action can transform the thin white line into a much broader, cirrus-like cloud layer that can cover significant portions of the sky.
These persistent contrails are meteorologically significant because they create new, thin, high-altitude clouds known as contrail cirrus. Like natural cirrus clouds, they can influence the Earth’s temperature by trapping heat radiated from the surface, a subject of ongoing climate study. The variability in trail persistence is a reliable indicator of moisture aloft, suggesting the atmosphere is primed for natural cloud formation.
Dispelling the Chemtrail Myth
The visibility of persistent contrails has led to the “chemtrail” hypothesis, which suggests the trails are chemical or biological agents being sprayed for nefarious purposes. Scientific consensus dismisses this theory, confirming that all observed persistent trails are normal contrails composed of water ice crystals.
The main argument from chemtrail proponents is that trails did not used to last as long as they do now, but this is factually incorrect. Historical records and photographs from as far back as World War II show examples of long-lasting contrails when the atmospheric conditions were right. The increase in global air traffic since the mid-20th century, however, has simply made persistent contrails a more common sight.
Furthermore, comprehensive surveys of atmospheric scientists have found virtually no evidence to support the existence of a secret, large-scale atmospheric spraying program. In one peer-reviewed study, 76 out of 77 atmospheric chemists and geochemists stated they had encountered no evidence of such a program. The single dissenter cited only ambiguous findings that were not conclusively linked to aircraft.
The trails that last for hours and spread out are persistent spreading contrails, hydrological phenomena made of ice. They are not distinct from short-lived contrails in their fundamental composition, only in their reaction to the ambient humidity and wind conditions at altitude. The scientific community, including organizations like NASA and the Environmental Protection Agency, consistently explains these trails as natural physical processes resulting from jet exhaust under specific weather conditions.