Good morning Mr Halsalll, Thank you for your email. As far as the Met Office is concerned there are only contrails - persistent and non-persistent. We do not recognise such things as chemtrails. We do realise there are others who do not agree and take a more sinister view that chemtrails exist and are causing damage to our atmosphere but we have no evidence to support that view. The exhaust from jet engines contains the by-products of the combustion of aviation fuel. These by-products contain aerosols (microscopic particles suspended in the air) and also water vapour (water in its gaseous form). In certain atmospheric conditions these aerosols and water vapour can enhance the likelihood of condensation taking place (hence contrails). Contrail formation is dependent on three major factors: the engine exhaust characteristics; the air temperature; and the humidity of the air. Aircraft condensation trails (“contrails”) are formed by mixing between the engine exhaust air and the surrounding environmental air. The exhaust air contains additional water vapour that has been released by burning fuel and the mixing process creates a plume of air that is briefly super-saturated with water vapour (i.e. contains more than enough water vapour to be saturated at its existing temperature). Observations of contrail formation conditions show that it is necessary to achieve super-saturation with respect to liquid water in order for cloud particles to be nucleated in the mixing plume. This theory is the same as that underlying the appearance of condensation in one’s breath on a cold day. With the current generation of airliner jet engines, contrail formation typically occurs at temperatures below about minus 45 °C, with some dependence on the humidity of the ambient air. At such temperatures, the cloud particles forming in the contrail freeze almost instantaneously to leave a cloud of small ice crystals. The temperature conditions required for contrail formation typically occur at altitudes of 30,000ft and above, the typical altitude for many airliners in cruising flight. They may occasionally form at slightly lower altitudes provided that the temperature and humidity conditions are appropriate. In a very dry atmosphere, continued mixing between the contrail, which is very turbulent, and the environment can lead to the air becoming sub-saturated, resulting in the cloud particles evaporating. Particle evaporation also takes place due to the descent (and hence adiabatic warming) as the contrail is entrained into the wingtip vortices. This produces a so-called non-persistent contrail which can be seen to dissipate at some distance behind the aircraft which generated it. In some circumstances, it is possible for the environmental air in which the contrail forms to be at or above the saturation humidity with respect to ice. In this circumstance, mixing between the contrail and the environment does not result in the mixture becoming sub-saturated with respect to ice. Hence, the ice crystals in the contrail can persist for long periods or even grow larger as they absorb the excess water vapour from the environment. This results in a contrail that can persist for many minutes or hours after the passage of the generating aircraft. This can be very noticeable, especially if it occurs near air traffic route “hubs” where many aircraft converge. Persistent contrails are not a new phenomenon. There is a large amount of still and moving photography showing persistent contrails produced by combat aircraft during the World War II. The High-Altitude Flight, which later became the Meteorological Research Flight, was formed in 1942 to investigate the conditions under which contrails formed. The part of atmosphere in which contrails form (the upper troposphere) can be a region in which strong wind-shear — where changes of wind speed or direction occurs with height. Growing ice crystals in the contrail can, therefore, fall into a layer of the atmosphere in which the wind speed or direction is different from that in its formation layer. This means that the ice crystals can then be carried horizontally away from the contrail. This contrail spreading is most apparent if the contrail is orientated perpendicular to the wind shear direction and can easily result in cloud streaks that may be many kilometres wide and visible in satellite cloud images. In regions of heavy air traffic, it is quite common for the spreading of persistent contrails to result in a thin overcast that covers most of the sky when viewed from the ground. Such overcasts can be difficult to distinguish from natural cirrus cloud. This is also formed in the upper troposphere and is composed of ice crystals. The Met Office has made observations of contrail initiation in the wake of the FAAM BAe146 research aircraft with a view to providing additional validation of the Schumann (1996) theory (see further reading below). The results of these observations were in close agreement with the theoretical predictions. The vast majority of global aviation is from commercial airlines. Military aircraft account for, by comparison, a very small portion of all contrails. The effects of persistent contrails can reduce the amount of the sun’s energy reaching the surface of the earth (the so-called “global dimming” effect). This effect is small but measurable and has a very small cooling affect. Contrails during the night however, trap the earth’s heat and so have a warming effect. The net effect is therefore difficult to establish. The carbon dioxide emission from the burning of aviation fuel has a much more significant impact on the climate. Carbon dioxide is a greenhouse gas and so aviation, along with all other fossil fuel burning transport forms, is contributing to human caused global warming. If you are interested in reading more please see the following further reading: Schumann, U.; 1996, On conditions for contrail formation from aircraft exhausts. Meteorologische Zeitschrift, vol.5, 4-23 Schumann, U., R.Busen and S.Plohr; 2000, Influence of propulsion efficiency on contrail formation: theory and experimental validation. Deutsches Zentrum fuer Luft- und Raumfahrt, Institut fuer Physik der Atmosphare Report No. 139. (copy available from National Meteorological Library, Met Office, Exeter, UK) Spichtinger, P., K.Gierens, and H.Wernli.; 2005, A case study on the formation and evolution of ice supersaturation in the vicinity of a warm conveyor belts outflow region. Atmospheric Chemistry and Physics, vol.5, Pp 973-987 I hope this helps to clarify the matter for you and thank you for taking the time to contact us with your concern. Kind regards, Emma Weather Desk Advisor Met Office, FitzRoy Road, Exeter, Devon, EX1 3PB, United Kingdom. Tel: 0870 900 0100 or +44 (0)1392 88 5680 Fax: 0870 900 5050 Twitter: @metoffice Email: [email protected] or visit metoffice.gov.uk
Posted on: Mon, 17 Mar 2014 14:18:49 +0000
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