Some social media users are claiming the long, white clouds left behind by aeroplanes are ‘chemtrails’ being sprayed into the air as part of a plot to poison humans or change the weather.
They pose no direct risk to human health but do contribute to climate change by preventing heat from leaving the earth.
It normally refers to beliefs governments or nefarious groups are secretly spraying toxic chemicals into the atmosphere.
While low-flying planes are used for crop dusting over agricultural land, the various versions of the conspiracy refer to commercial aircraft high in the sky.
Some claim chemtrails are being used to pacify or reduce populations.
The COVID pandemic even produced claims chemtrails are vaccines being sprayed to inoculate people.
Like many conspiracy theories, false beliefs are based on kernels of truth mixed with large chunks of misinformation.
Between 1940 and 1979, Britain’s Ministry of Defence secretly tested biological agents on parts of the UK during military germ warfare trials.
Similar tests took place in the US, including the release of bacteria in 1950 that sent 11 people to hospital.
Modern conspiracies rely on the idea chemicals are sprayed constantly across the globe in full view of people on the ground.
As this Harvard University research group points out, a program of this scale would require “tens of thousands” of workers to manufacture and disperse the alleged materials.
It’s also true cloud seeding has been used in attempts to boost winter snowfalls or alleviate droughts.
The truth is a lot simpler.
The water condenses, turning into ice particles, with planes flying in cold and humid conditions leaving behind trails of condensation resembling long, thin clouds from each engine.
As an aircraft moves through the air, the water condenses on nitrogen and sulphur oxides in the exhaust to form new particles, Dr Schofield says.
The visibility of contrails greatly depends on weather conditions, as explained in this 2021 article from David Simon Lee, a professor of atmospheric science and aviation at Manchester Metropolitan University.
They can last for hours in cold and humid air at high altitudes but quickly disappear in low-humidity conditions.
Andrew Carleton, a professor of physical geography and climatology at Penn State University, says the shape and visibility of contrails also depended on factors such as wind conditions and flight paths.
“Also, the greater the number of flight paths in a given region (such as the midwest USA, southeastern Australia), the greater the likelihood of multiple persisting contrails because of the additional particulates and water vapour emissions,” Prof Carleton told AAP FactCheck.
He said the idea contrails were used to intentionally spread chemicals from commercial aircraft was fanciful.
“As a possible way to try and achieve some kind of ‘mind control’, contrails would be a decidedly inefficient one, only occurring when and where atmospheric conditions are favourable and in association with commercial flight paths,” Prof Carleton said.
In 2016, US researchers tackled the chemtrail myth, surveying 77 atmospheric chemists and geochemists with expertise in condensation trails. All but one said they had seen no evidence of chemtrails.
The researchers said contrails could be lasting longer in the sky due to changes in aircraft technology, but “the evidence as evaluated here does not point to a secret atmospheric spraying program” (Page 9).
However, contrails have an indirect impact on the world’s climate because the clouds they form are good at trapping heat.
They are estimated to account for more than half the total climate impact of aircraft, although an IPCC report says contrails only make up a small proportion of overall greenhouse gas emissions (Page 7).
Claims the long, wispy clouds left behind by planes are toxic chemicals and biological agents known as “chemtrails” are false.
The clouds are non-toxic condensation trails created by ice particles formed when water vapour condenses in a jet engine exhaust.
False – The claim is inaccurate.