Contrails - Research, comments and links

Contrails and Aviation cirrus

General and Definitions (02)

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Airplane condensation trails, or contrails for short, form when aircraft fly in an environment where ice particles nucleate from the water vapor in the jet exhaust. These conditions often occur simultaneously with the presence of cirrus clouds. To further complicate their identity, contrails often spread horizontally and vertically with time such that they become indistinguishable from cirrus clouds.
(Link to, no longer available.)

If you live near an airport, you probably see a special type of cloud called a contrail all the time. Contrails are cirrus-like clouds that form from jet exhaust. The hot, moist exhaust from a jet engine mixes with the cold dry air in the upper troposphere, causing condensation. Contrails appear as white streaks across the sky and follow the jet's path.
(Link to no longer available.)

Op 21 en 22 mei waren op uitgebreide schaal sluierwolken te zien voor een deel veroorzaakt door vliegtuigen. De hardnekkige vliegtuigstrepen hingen samen met een vochtige luchtlaag op ongeveer 8 kilometer hoogte en mengden zich met de hoge bewolking die van nature aanwezig was. Door het binnenstromen van drogere lucht op 23 mei is de hoeveelheid hoge sluierbewolking op sterk afgenomen en in de diep blauwe hemel lossen ook de vliegtuigstrepen snel op.

What is a contrail and how does it form?

To answer this question, lets first identify what a contrail is. A contrail is the condensation trail that is left behind by a passing jet plane. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. Vapor pressure is just a fancy term for the amount of pressure that is exerted by water vapor itself (as opposed to atmospheric, or barometric, pressure which is due to the weight of the entire atmosphere above you). The mixing occurs directly behind the plane due to the turbulence generated by the engine. If condensation (conversion from a gas to a liquid) occurs, then a contrail becomes visible. Since air temperatures at these high atmospheric levels are very cold (generally colder than -40 F), only a small amount of liquid is necessary for condensation to occur. Water is a normal byproduct of combustion in engines.

This cloud formation is very similar to the process that occurs when you breath on a cold winter day and you can see your own breath in the form of a "cloud". You may have noticed that on some days this "cloud" you produce lasts longer than on other days where it quickly disappears. The length of time that a contrail lasts is directly proportional to the amount of humidity that is already in the atmosphere. A drier atmosphere leads to a more short-lived contrail, while an atmosphere that has more humidity will lead to longer-lived contrails. However, if the atmosphere is too dry, no contrails will form. Occasionally a jet plane, especially if ascending or descending, will pass through a much drier or more moist layer of atmosphere which may result in a broken pattern to the contrail, with it appearing in segments rather than in one continuous plume.

How can I find out more about contrails?

There is a lot of information on the internet regarding contrails and some of the studies that are ongoing relating to research attempts at better understanding contrails. Those links provided below are only a few of the locations elsewhere on the web where you can find more information regarding contrails.

  • CIMSS Contrail Web Page (Link no longer valid)
  • NASA-Study regarding contrails as seen with lidar (link no longer valid)
  • NASA-The Atmospheric Effects of Aviation Project  
    (At the direction of Randy Kawa, the AEAP web pages have been removed.)
  • Atmospheric Radiation Measurement (ARM) Program


Making Clouds by Flying in Airplanes

Contrails are cirrus clouds triggered by high flying airplanes. As other, natural clouds they influence net radiation and climate. To quantify their influence on climate it is necessary to estimate the amount and coverage of contrails and to determine the optical properties of contrails under variing conditions.

The word contrail is short for (jet) aircraft condensation trail. Contrails can cause a day to be cloudy or overcast that would otherwise have been clear-sky. The presence of contrails is an indication of high moisture in the upper region of the troposphere. Part of a contrail freezes, while the other part descends like mammatus and evaporates. The frozen remnant of the contrail has the appearance of a thin line. As this is still drifting backwards by the jet engine exhaust velocity, movement of the contrail will become unstable and will begin to show some curvature. Eventually the frozen contrail breaks up into parts, sometimes forming "smoke" rings (vortices with the ends connected). At other atmospheric states, the contrail will transform into cirrus or cirrocumulus

Contrails form through the injection of water vapor into the atmosphere by exhaust fumes from a jet engine. If there is sufficient mixing between the cold upper tropospheric air and the hot exhaust gases to produce a state of saturation, ice crystals will develop. Even tiny nuclei released in the exhaust fumes may be sufficient enough to generate ice crystals.

Contrails are Bad News