Contrails or Homomutatus are long and thin artificial clouds that especially in the wake of aircraft can be caused by water vapor and other gases, as well as vacuum. In particular, they are typical and permanent for altitudes above about 8 km when engine exhaust gases containing water vapor and soot meet relatively cold air. They can develop and continue to exist in otherwise cloud-free areas if there are no condensation nuclei for natural cloud formation. They belong to the cirrus group and also represent an important class of anthropogenic clouds. In humid air they can also occur at lower altitudes. There they can consist of droplets instead of ice crystals.
Origin and composition
At the cruising altitude of long-haul jets, it is below −40 ° C, so that contrails form even in relatively dry air. In principle, gaseous or freezing liquid components of the air and exhaust gases condense or resublimate (explained in more detail under sublimation ), promoted by the simultaneous appearance of soot particles from the exhaust gas, which serve as condensation nuclei or crystallization nuclei . In these ambient conditions, nucleation and thus condensation / freezing can also occur spontaneously from local density fluctuations , i.e. H. arise without a nucleus or nucleus. When swirling with cold ambient air, the saturation vapor pressure decreases much more than the partial pressure of the water, with the result of supersaturation . The soot particles in the exhaust gas allow the rapid formation of nuclei as water molecules attach to them. Ice crystals are formed directly at low temperatures .
The main products of combustion of fuel are carbon dioxide and water vapor , as well as in smaller proportions soot, sulfuric acid , nitric acid and the combustion and recombinant products of fuel additives . Kerosene is not a pure substance with a uniform composition, but a mixture of various substances which, when combined, make it possible to use it as a fuel for these extreme requirements (see also JP-8 or the numerous fuel specifications for military aviation ).
Air pollution that is already in liquid or gaseous form in the air, such as freons or other solvents or natural terpenes, can also be sucked in for combustion under these conditions or condense or resublimate in the exhaust gas flow from the ambient air.
Up to a size of about 100 nanometers , the crystals hardly scatter light, then primarily blue light at first. Only when additional water particles accumulate do they reach a size in which they scatter light regardless of its wavelength and become bright white. That and the remaining waste heat explain the characteristic gap between engines and contrails. The stripes increase in width and in four-jet jets initially touch in pairs. As the air in the lower part of the wake vortex diverges , the gap in the middle initially becomes wider and the stripes from the internal thrusters disappear under the outer jets. Further turbulence creates a single wide strip that sinks overall.
Persistence and Change
The further course depends heavily on the situation, in particular on the relative humidity . In around 70 percent of the cases, the air is undersaturated, the relative humidity is below 100 percent, and the contrails dissolve within a few minutes. The vertical extension is then 300 to 500 meters, depending on the type of aircraft.
If the humidity is around 100 percent, the contrail dissolves because the relative humidity decreases as it drops. How quickly individual volume elements become invisible depends on the particle size and their original position in the beam. It can mammatusähnliche form protuberances.
If the ambient air is over-saturated, the contrails persist for a longer period of time. Humidity levels of over 200% are possible at high altitudes. The amount of water vapor absorbed from the atmosphere can then exceed the engine output by several orders of magnitude. The lifespan can be several hours, in one case a single contrail was visible over 17 hours on a satellite image. Depending on the prevailing wind shear , the width of the contrails can grow to over 20 km; they are then only difficult to distinguish from naturally formed cirrus . In the professional world is then spoken of contrail cirrus . These can remain in the sky for several days. Most of the time, however, they soon dissolve due to a large-scale lowering of the air or change into a closed cloud cover due to large-scale uplift.
If an aircraft flies close above or below a thin layer of cloud, the downdraft caused by it can dissolve the cloud. The contrail can also cast a shadow on a thin layer of cloud below, which also results in a dark streak.
Furthermore, contrails can also become visible at night when they absorb or scatter the moonlight and thereby partially cover the moon.
Effects on the climate
Air traffic influences the climate through the emission of carbon dioxide and nitrogen oxides as well as through the formation of contrails. The anthropogenic contrails cover a small part of the sky and thus reduce solar radiation during the day through reflection on their upper side (cooling effect) and thus increase the planetary albedo (see cloud ). On the other hand, ice crystals absorb the radiation coming from the ground and re-emit less high-energy radiation ( greenhouse effect ), which causes warming. It is therefore believed that the climate is influenced by the contrails of air traffic . The strength of this effect and its role in relation to global darkening or global warming are known so far only with great uncertainty, but an influence on global radiation of up to 2 W / m 2 is estimated locally . Line-shaped contrails cover an average of around 0.5% of the sky over Central Europe, 0.7% during the day and just under 0.3% at night. The contrail cirrus, which is difficult to measure, is not taken into account and there are indications that the degree of coverage of all contrails is much higher. A DLR study found that the contrail cirrus clouds over Central Europe can temporarily cover up to ten percent of the sky.
At 31 mW / m 2, the warming of the earth's atmosphere through contrail cirrus is somewhat greater than the effect of the emitted CO 2 . The radiative forcing of contrails alone is even exceeded by a factor of nine by contrail-induced clouds. With this knowledge, the impact on climate change could be reduced by simple measures - for example by flying around particularly humid areas (whereby the associated additional emissions must be taken into account) or by making modifications to fuel or engines so that the emissions of soot and water vapor can be reduced .
The other aerosol particles in the aircraft exhaust can also change the natural cloud formation over days and in a comparatively large area.
Contrails from rockets
When rocket fuels are burned, too - depending on the type of fuel - water vapor and possibly also solid components such as soot are generated. The boosters of solid rocket rockets mainly contain ammonium perchlorate and aluminum , from which very dense aerosol strips of hydrochloric acid and aluminum oxide are formed at all altitudes . Because of the mostly vertical flight path and the effect of the wind, contrails from rockets show a strong dependency on wind direction and wind strength . This often results in a zigzag course that no longer corresponds to the actual flight path.
Vapor trails from ships
The exhaust gases from large ship engines can also leave long plumes of condensation close to the ground.
Condensation through negative pressure
In moist air, a sharp drop in pressure can quickly lead to visible condensation. Above the wings of airplanes and behind the shock front emanating from supersonic aircraft, see cloud disc effect , the fog dissolves again immediately. In the core of tip vortices , however, the negative pressure lasts longer, so that longer contrails can arise there.
- Hail fliers , influencing the weather by spreading certain substances in clouds
- Chemtrails , a conspiracy theory about purposely chemical-infused contrails
- Steam locomotives as well as heating and combustion power plants develop slightly rising plumes of condensate
- Exhaled air and vehicles with internal combustion engines lead to clouds of condensation if the air temperature is sufficiently cold, but these do not last much
- Environmental impact of aviation
- The climate impact of contrails is significantly lower than expected - but more cirrus formation from air traffic. Intergovernmental Panel on Climate Change (IPCC) and DLR, November 28, 2005, accessed December 9, 2009 .
- Warming night flights. On: Wissenschaft.de of June 15, 2006. - Report on a publication in Nature (Vol. 441, 2006, p. 864, doi: 10.1038 / nature04877 )
- Unterstraßer, Simon: Numerical simulations of contrails and their transition into cirrus clouds (abstract; PDF (18 MB) downloadable) - PhD thesis at the LMU Munich, Faculty of Physics (2008)
- Contrails are now called Homomutatus
- vapor trail. In: Encyclopædia Britannica. Encyclopædia Britannica Inc, 2012, accessed April 17, 2012 .
- VDI 3491 sheet 4: 2018-03 measurement of particles; Manufacturing process for test aerosols; Condensation methods (Measurement of particles; Methods for generating test aerosols; Condensation methods). Beuth Verlag, Berlin, p. 4.
- Joachim Curtius: Aerosol sulfuric acid in the atmosphere and in the wake of jet aircraft: Development and use of a new, airborne mass spectrometer probe
- Dominik Schäuble: Construction of an airborne mass spectrometer for measuring HNO3 and HONO and quantifying the HNO3 uptake in ice particles in contrails and cirrus clouds ; Mainz, 2010, (PDF file)
- P. Minnis, et al .: Transformation of contrails into cirrus during SUCCESS . In: Geophysical Research Letters . 25, No. 8, 1998, pp. 1157-1160. doi : 10.1029 / 97GL03314 .
- Climate factor contrails: Effect greater than the CO 2 emissions of the aircraft. In: Deutschlandfunk , March 31, 2011
- Global warming through contrail cirrus. DLR , March 30, 2011
- Estimated climate effect of contrail cirrus clouds. DLR , April 2011
- Shadow of a contrail ( memento of April 5, 2017 in the Internet Archive ) at APOD
- Ercan Kayaoglu: DLR - Institute for Atmospheric Physics - Contrails. Retrieved April 13, 2017 .
- Ercan Kayaoglu: DLR - Institute for Atmospheric Physics - Contrails. Retrieved January 23, 2017 .
- Roger Teoh, Ulrich Schumann, Arnab Majumdar, Marc EJ Stettler: Mitigating the Climate Forcing of Aircraft Contrails by Small-Scale Diversions and Technology Adoption. In: Environmental Science & Technology. 2020, doi : 10.1021 / acs.est.9b05608 .
- DLR : image "Contrails from ships off Brittany" ; Associated caption "Vapor trails from ships off Brittany"
- Engl. Wikipedia: " Ship tracks "
- Ship contrails off the US east coast. In: earthobservatory.nasa.gov