Contrail effect by Daryl O’Dowd CONTRAILS VERSUS CHEMTRAILS An interesting take on aircraft ice crystal plumes As air traffic increases, contrails are having a greater and greater effect on the Earth’s weather, and there are some interesting advantages to the phenomena

ondensation trails are not a natural vapor can condense. Aircraft engines, phenomena. Copious quantities of turbine engines in particular, provide both C water vapor and particulate matter in quantity. For every kilo of fuel consumed, are needed to cause the condensation- 1.4kg of water vapor can be produced. A Above: Kerosene combustion. Shown are the freezing processes that result in an ice crystal Boeing 747 at cruise speed and altitude will ideal products for the burning of dodecane plume behind an aircraft at high altitude. have a typical mid-flight fuel burn of 21,800 – a common kerosene component. For Fuel additives likely impact the lb/hour and will eject approximately 6.1 lb each kilo burned, up to 1.4kg of water vapor hygroscopic nature of carbonaceous exhaust (2.75kg) of water vapor a second. is produced particles and may increase or even decrease The exhaust plume also contains the efficacy of the process. Minimizing or incompletely burned hydrocarbon particles At its simplest, where the local mitigating contrails would be a tactical (soot) that encourage the phase change of atmosphere at the aircraft level is considered advantage in military operations to prevent the engine exhaust water vapor, first to to be humid – this threshold varying with observation of aircraft. From a civilian or liquid then to ice. Initially these particles act temperature, dew point, and pressure commercial perspective, contrails allow as cloud condensation nuclei (CCN) and (altitude) – then a contrail will persist. It ready identification of aircraft in high level provide a site for water vapor to condense as will grow and spread as water vapor in the flight and potentially reduce the risk of the exhaust plume entrains environmental surrounding natural atmosphere deposits mid-air conflict. While spreading contrails air and begins to cool. directly on the ice substrate. Where the do reduce incoming shortwave radiation Almost immediately these droplets freeze atmosphere is dry – often seen during much like natural cirrus shields, currently into ice crystals, the limit for supercooled widespread subsidence associated with a less than 0.2% of the earth’s surface is liquid water being approximately -40Cº. An stagnant upper ridge or broad high pressure contrail-affected, suggesting that the global aircraft operating at warmer temperatures zone (e.g. Azores High) – the ice in the energy balance is negligibly impacted by may initially generate a liquid droplet plume will rapidly sublimate and the aircraft contrails. condensation trail, but as the vapor pressure contrail will end a wingspan or two behind In 1996, a think-tank paper entitled over liquid water is greater than over ice, the the aircraft. Weather as a Force Multiplier: Owning the plume will preferentially form ice crystals at Weather in 2025 was written for the expense of water droplets. Fuel, additives, and soot consideration by the US Air Force (USAF). Whether these ice crystals – proto-cirrus The majority of turbine powered aircraft The 44-page treatise made numerous particles – persist is a function of the flying at altitude use either a kerosene-based projections as to how military weather environmental temperature and humidity at fuel (Jet A, Jet A-1, JP-8) or a naphtha- control would be executed, including the the point of injection. Critical to concealing kerosene blend (Jet B). In addition, and use of UAVs at high altitude to release military or stealth aircraft, especially during potentially important in the effectiveness of chemicals to alter cloud and airmass the day, contrail formation was initially soot as CCN, are fuel additives designed to properties. The term chemtrail (chemical investigated in Germany in 1941, and later reduce corrosion, icing, static build-up, and trail) entered the public lexicon to describe detailed tables and charts (still in use) were fungal or bacterial growth. The efficacy of the such atmospheric tampering and became a created for the USAF in 1953. soot particles in first acting as CCN, catalyst for public concern over any capturing water vapor, and then second as aircraft-produced trail in the atmosphere. Contrail formation. Engine water vapor potential ice nucleator, facilitating the further condenses on soot particles and then freezes as conversion of supercooled droplets to ice. Contrail formation process entrained air cools the exhaust plume This may be a function of the additives Condensation trails (or contrails) are water/ imparting surface characteristics on the soot. ice plumes stretching behind any aircraft – jet or propeller – operating at flight levels where the ambient temperature is typically -40ºC or colder. Critical to the creation of contrails, brief or persistent, is the need to have a source of water vapor as well as a physical substrate onto which the water

72 • METEOROLOGICAL TECHNOLOGY INTERNATIONAL AUGUST 2012 Contrail effect

“Following the 9-11 attacks when all civilian aircraft in the USA were CLOUD grounded… aircraft-produced SEEDING cirrus decreased” Commercial cloud seeding operations have been undertaken since the 1950s. Currently, projects to increase rain and snowfall or suppress hail are executed in more than 20 countries, including France, the USA, Canada, Greece, Thailand, Argentina, and Australia. Some projects use low-cost, ground-based systems to seed wide areas, often for winter snow enhancement. Others use aircraft to target specific clouds – flying in the updrafts at cloud base or within growing convective towers – to suppress hail. However, no commercial seeding is undertaken at contrail altitudes.

aircraft-produced cirrus decreased. The transition from a handful of contrails to a full covering of cirrus, often within a few hours, can appear suspicious to the layman unaware that water vapor is not uncommon at higher altitudes.

Red contrails and absence Illuminated from below by deeply scattered dawn or dusk light, persistent contrails can have a strong red hue and suggest that chemical injection is being undertaken. Viewed from above, however, these same contrails would appear grey or white. The presence of contrails on some days Contrail illumination. Illuminated from below at and their complete absence on others – twilight, the red hue imparted on contrails entirely the consequence of natural mistakenly conveys the impression that chemical atmospheric variations of humidity and additives are being released. (Courtesy USAF) temperature in both the vertical and Cloud grids horizontal – mistakenly suggests an ability As the number of aircraft movements to control the contrail effect at will. increases, so does the potential for Aircraft moving on west-east flight Condensation trails have been present contrails. Daily, there are more than corridors inevitably must intersect in the atmosphere for more than 75 years. 85,000 aircraft flights within the USA and north-south traffic routes. When conditions However while cirrus, and aircraft- as many as 30,000 IFR movements within support contrail formation, the resulting produced cirrus in particular, can effect the Eurocontrol zone. Commercial aircraft intersected contrail symmetry can be very the local radiation balance, it is estimated are usually constrained to follow conspicuous. When the airmass is also that less than 0.2% of the globe is designated high-level airways or a limited moving, new contrails are continuously contrail-affected. While contrails may be number of North Atlantic Tracks (NAT added in a fixed location, but drift considered to be a nuisance by the Tracks) and it’s not unusual for busy downwind resulting in parallel cloud military, they may have a civilian cockpit airspace intersections to have 70 or 80 tracks or an interlaced cloud grid. To the safety value in calling attention to nearby aircraft passages per hour. On airways layman unaware of flight routing, this aircraft in flight. To the meteorologist with reduced vertical separation minima symmetry may suggest that a deliberate they offer a simple tool to estimating (RVSM) in place (FL290 to FL410) the cloud-seeding exercise is underway. upper atmosphere moisture advection, aircraft density can be even higher. Where humidity is high, contrails, even as well as high-level wind speed and This concentration of aircraft at altitude isolated ones, may naturally grow from direction systems. z has given rise to a number of contrail cloud background atmospheric vapor and phenomena – cloud grids, spreading cirrus gradually spread across the entire sky. This Daryl O’Dowd is a Canadian Meteorological and shields, red plumes, and sudden contrail was demonstrated in the 72 hours Oceanographic Society accredited wind energy, radar, absences – which have inadvertently fueled following the 9-11 attacks when all civilian and weather modification meteorologist, based in chemtrail speculation. aircraft in the USA were grounded and Calgary, Alberta, Canada

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