Wind farms and fuel additives possible catalysts in the generation of spreading aviation contrails   by Dr Chris Barnes, Bangor Scientific and Educational Consultants, July 16th 2014.   





Since approximately 1997, sky watchers have noticed dramatic changes in the behaviour of aviation contrails, particularly those under busy air-lanes.   The period has seen contrails which spread out much more into elaborate shapes and patterns which can under some conditions persist in the sky and even transform into cirrus cloud.  Rarely do contrails remain as the fine pencil line of old.   The present author has also noticed contrails which spread out and produce fall-streaks over part of the sky and then leave gaps with no contrail at all and then leave line contrails towards the horizon.  These days the sun often looks more ‘halogenic’ as well.  This together with the observation of more cases of crepuscular rays and other rare optical phenomena    is an indicator of more aerosols in the atmospheric environment and a greater range in size and shape distribution in water droplets and ice crystals.  Of course there will be others who never look up and have noticed no significant difference in our skies. 


The author has previously advanced hypotheses as to the cause of spreading and persistent contrails and has discussed their weather and climate implications (   For example;  contrails may form easier and persist longer due to new types of aircraft engine, the so called high bypass ratio, see Schumann (2000) and more advanced aviation fuel with flame retardant and cleanser/lubricant properties, see below.  Also high sulphur fuels cause longer contrails with more tendency to persist, see Schumann et al (1996) and there is a threshold below which changing  fuel sulphur concentration makes no discernible visible effect, Busen and Schumann (1995).     Aircraft density in the major flight corridors is much higher so there is much more ice nucleation material, sulphate and black carbon present.  Aircraft can also fly closer together due to wave avoidance technologies.   Contrails are always likely to form in aircraft wakes,  All these factors affect the type and number density of contrails formed.   Also because of AMDAR (weather data collection technology employed on aircraft)  they are required to make small, discrete, jumps in cruise altitude. This too may change the form and shape of contrails.  AMDAR has included information on atmospheric super-saturation since 2006 (see Gierens et al 2008) so may has the potential to be used in contrail avoidance strategy.  However, the purpose of this paper is to consider if  wind farms are the main catalyst  or at least an additional catalyst  in the generation of such new families of contrails. Jet fuels also contain mixtures of antioxidants, detergent/dispersants, and metal deactivators, see Zarbarnick and Grinstead (1994). In more detail;  Additives may be included in aviation fuels to improve fuel performance - generally by eliminating undesirable effects - or to meet specific requirements of certain aircraft or airline operators. They are added in quantities that are often only measurable in parts per million.


The additive content of jet fuels varies considerably, depending on whether the fuel is for civil or military use. Additive packages for Avgas, on the other hand, are fairly standard. Only additives that have been through a detailed and comprehensive evaluation and approval process are permitted.


Tetra-ethyl lead is added to improve the anti-knock characteristics of Avgas. There are pressures to eliminate this additive on environmental grounds. Alternative ways of boosting the octane rating of Avgas have still to be found however.


Antioxidants (gum inhibitors) must be used in Avgas to prevent the formation of gum and other antioxidation products. Jet fuels, which are inherently more stable than gasolines, may contain them, depending on the treatment process used during manufacture.


A metal deactivator - now rarely used - may be added to nullify the effects of dissolved trace metals, especially copper, which can impair the thermal stability of jet fuels.


Corrosion inhibitors can be used to reduce corrosion in fuel system and improve the lubricity - lubricating properties - of jet fuels.


Fuel system icing inhibitors reduce the freezing point of any water that may be in the fuel system and prevent the formation of ice crystals that could restrict fuel flow. They are mandatory in military Jet but are not used in civil aircraft that have fuel heaters. They may be added to either Jet or Avgas during the fuelling of small aircraft.


Static dissipater additives minimise the hazardous effects of static charges that build up during movement of jet fuels. They are sometimes used in Avgas as well, see Shell Global Website.   Since the precise balance and composition of these may vary from one type of fuel to another, this too could potentially impact form and ice crystal size of contrails especially if aerosol is released  but there appears to be nothing documented in the scientific literature.      Although  many aviation fuel manufacturers prefer to list their fuel additives as 'trade secret', this is extremely unhelpful to climate and atmospheric scientists.  An extensive 'data mining' type  search of the scientific and patent literature suggest that ammonium,  calcium aluminium  and barium salts of dinonylnaphthalenesulfonic acid would be the types of compounds used for corrosion inhibition and static dispersion.  Dinonylnaphthalenesulfonic acid has a micelle like structure and can be used for extracting metals from aqueous organic mixtures, see      amongst others   Such are 'Thermooxidatively' stable compositions, see   In preventing static build up and corrosion in aircraft engines, metal complex organic micelles will be released to the atmosphere. It has recently been proposed that natural aerosols are  organic aerosols in  an ‘inverted micelle’ form consisting of an aqueous core that is encapsulated in an inert, hydrophobic organic monolayer. The organic materials that coat the aerosol particles are surfactants of biological origin, see Barney Ellison et al (2012).  Kulmala et al (2004) have postulated that biogenic aerosol cools the planet in an negative feedback  response to  increasing CO2,   Possibly the aerosol being inadvertently released by the modern aircraft fleet  may act in a way similar to that of natural organic aerosol. If this is the case ways could be conceived to enhance the process if SRM or large scale geo-engineering were ever to be required.   


Contrails and Wind Shear

Lewellen and Lewellen (2001) have shown the fraction of the initial contrail ice crystals surviving, their spatial distribution, and the ice mass distribution are all sensitive to the aircraft type, ambient humidity, assumed initial ice crystal number, and ambient turbulence conditions and that there is a significant range of conditions for which a smaller transport such as a B737 produces as significant a persistent contrail as a larger transport such as a B747, even though the latter consumes almost five times as much fuel.  Contrail horizontal extent is known to increase due to the wind shear, see Jensen et al.  Duda et al (2004) showed that persistent contrail formation density was roughly correlated with air traffic density. Liou and Ou reached a similar conclusion for the abundance of ‘high cloud’. 


Can wind farms influence contrails?

Wind farms are both sinks of energy and sources of turbulence, see Roy 2004.

Large-scale use of wind power can alter local and global climate by extracting kinetic energy and altering turbulent transport in the atmospheric boundary layer, see Kieth et al, who also showed that added turbulence intensity occurred downstream of a wind farm is found for 7 of the 19 cases.   Van den berg (2007)   states that   atmospheric stability cannot, with respect to modern, tall wind turbines, be viewed as a ‘small perturbation to a basic neutral state’.   Barnes ( ref) has already suggested that wind farms may be causing freak rainfall events in the uk.    Wang and Prinn  (2010) suggest  that wind farms  will cause alterations of the global distributions of rainfall and clouds.  Since contrails are specialised clouds it is logical to suggest that these too will be affected.  Barrie and Kirk –Davidoff (2010) calculate   significantly impact  on the atmospheric circulation due to the additional surface roughness forcing. This forcing could be changed deliberately by adjusting the attitude of the turbines. Induced perturbations involve substantial changes in the track and development of cyclones over the North Atlantic, and the magnitude of the perturbations rise above the level of forecast uncertainty.  Clearly if storm tracks and circulation are potentially so dramatically affected then so would  be provision of CCN for contrails and contrail cirrus.   Wind farms also affect the hydrometeorology of an area up to 18–23 km downwind, see Roy 2010. Large-scale deployment of wind power may alter climate through alteration of surface roughness. Previous research using GCMs has shown large-scale impacts of surface roughness perturbations but failed to elucidate the dynamic mechanisms that drove the observed responses in surface temperature. Using the NCAR Community Atmosphere Model in both its standard and aqua planet forms, the authors have explored the impact of isolated surface roughness anomalies on the model climate. A consistent Rossby wave response in the mean winds to roughness anomalies across a range of model implementations is found. This response generates appreciable wind, temperature, and cloudiness anomalies. The interrelationship of these responses is discussed, and it is shown that the magnitude of the responses scales with the horizontal length scale of the roughened region, as well as with the magnitude of the roughness anomaly. These results are further elucidated through comparison with results of a series of shallow-water model experiments, see Kirk Davidoff and Kieth.  


Seismic noise, infra-sound and AGW

Wind farms are also large emitters of seismic noise, see Auld et al (2014). They will effect waves of the ocean if offshore and emit infrasound.   A natural source of infrasound is microbaroms or sea wave signals at approximately 0.2 Hz and 120dB.  Microbarom wind wave interaction can be two way and involve all levels  of the troposphere, stratosphere and thermosphere, see amongst other Garces 2004. Microbaroms provide a natural atmopsheric probe, see Donn and Rind (1971). AGW is one coupling mechanism for processes in the various parts of the atmosphere.  Caveleri et al (2012)has extensively addressed the problem in 'Wind Waves in the Coupled Climate System'. Gregory (2009) has discussed the rare phenomenon of helical contrails over  the sea.  The present author has often observed such 'DNA like' trails in Bangor Wales which is a coastal city but feels that wind farm turbulence could also explain them.  


Wind farms emit infra-sound comb spectra as harmonics  of the blade crossing frequency. This infra-sound has been detected 60km from wind farms, see Pepyne Klaiber (2012)(  Although,perhaps, some 30 dB down   on microbaroms . Liska has also observed long range 2Hz infrasound from hydro-power  stations (1974). Kulichkov (2004) discussed long range mesopheric sound propagation in detail.  Tidal components have been observed by radar at Stratospheric and Mesopheric heights, see Woodman and Guillen (  1974).  There is thus no reason to suppose that Wind Farm infra=sound components would not be present at or below these heights, say the UTLS for example where many contrails are found. 

Arrowsmith et al (2010) describes the Siesmo-Acoustic wave-field as a new paradigm in the study of geophysical phenomena and several natural and anthropogenic sources are mentioned but  wind farms are perhaps erroneously omitted?

Contrails spread in upper atmosphere wind shear  which is also the mechanism for infra-sound  refraction, see Georges and Beasley (1977).  It stands to reason therefore that wind turbine infra-sound will alter the shape and form of contrails.  


Is this  new family of contrails 'planet friendly'

There is evidence to suggest that many forms of  contrails cool the planet by day and warm it by night.  There is further evidence to suggest that NOx injection by aircraft may also produce a somewhat unexpected cooling effect.  Presumably if the contrails are spread out more as a result of propagating disturbances from wind farms this may add to the predicted cooling effects.   However all other aircraft emissions are highly undesirable either because of their climate warming effects or because of their potential to be environmentally toxic.    


Testing the wind farm hypothesis

Clearly not all the world's wind farms and flight corridors correspond but it would be instructive to look at those which do and then to contrast contrail coverage in heavily used flight corridors where there are no wind farms. A crude visual geographic meta-analysis   can readily be done by using public domain mapping take from the internet.   

The world distribution of major wind farms is shown on the left.  Contrail coverage is shown on the right together with the combined effect of increasing aircraft efficiency i.e. higher by-pass ratios.  Clearly the east /west US/Europe routes are the most heavily used.  The hypothesis would appear to be supported in that  in every individual country  the contrail coverage  increases significantly over land where wind farms are in operation.  Of course, to some extent,  an abundance of internal flights in these regions in addition to international flights cannot be ruled out as being equally causative and further research will be required.        



Some evidence has been provided that wind farms may modify aircraft contrails but further work remains.  Similarly aviation fuel additives may be crucial in altering the types and balance of atmospheric  aerosol. In the interests of scientific understanding and possibly for all occupants of this planet it would be useful if fuel manufacturers would disclose the chemical composition of all of their additives.        If flying is restricted to the daytime it may be more climatically friendly.   In a changing global environment  no stone should be left unturned to make sure that there are no unpleasant or irreversible climactic  surprises   lurking in the near future.