Why do aircraft leave contrails in the sky?

These wispy man-made clouds have fascinated humans for a century, but what actually are they?

In 1915, around 12 years after the world's first controlled powered flight, Austrian physicist Robert Ettenreich was watching an aircraft skirting across the sky in Southern Tirol when he noticed something – a long wispy cloud trailing out behind it.

Ettenreich described what he saw in a 1919 paper as "the condensation of a cumulus stripe from the exhaust gases of an aircraft", noting that it was visible for a "long" time.

Aircraft had begun to reach the heights conducive to the formation of contrails. Others also began noticing the clouds trailing behind aircraft. A poetically inclined captain in the US Army Medical Corps described them in 1918 as "several strange and startling clouds – long, graceful, looping ribbons of white". "[N]ever before had I seen a plane writing in white upon the blue slate of sky," he added.

As this young aviation industry matured and exploded across our skies, contrails have become an increasingly common sight. Their eye-catching presence has caught the attention of many, including through the surprisingly common belief in the chemtrails conspiracy theory (more on this later). However, it's actually their climate impact that scientists are concerned about. So what actually are they – and should we be paying them more attention?

Following the initial observations in the early 20th Century, the exact cause of these plane-trailing clouds remained a topic of debate for several decades, says Ulrich Schumann, professor of atmospheric physics at the German Aerospace Center (DLR). Early theories proposed they were vibrations from the plane's engine or effects from electrical charges. Some did consider whether they occurred due to super-saturated water vapour, but this was dismissed as it was thought that too little water vapour was emitted.

Figuring out the causes of contrails didn’t become much of a concern until World War Two, when contrails were first viewed as an issue. "They make aircraft visible, you can see the trace of a flying aircraft," says Schumann. "So during the Second World War, militaries tried to avoid contrails because they wanted to avoid visibility of their aircraft."

The first correct explanations of how they formed were derived in the early 1940s, and 50s however, with what is now known as the Schmidt-Appleman criterion showing threshold conditions depended on ambient pressure, humidity and the ratio of water and heat released by the plane.

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In a nutshell, contrails – a contraction of "condensation trails" – are the line-shaped ice particle clouds formed in the wake of aircraft. They can be anything from a 100m (330ft) to several kilometres long.

Three things are needed for them to form: water vapour, cool air and particles on which the water vapour can condense. Water vapour is produced by planes as the hydrogen in its fuel reacts with oxygen in the air. In cold conditions (typically below around -40C (-40F)) it can condense, typically on the soot particles also emitted from aircraft engines, to a fog of droplets, which then freeze to form ice particles. The process broadly resembles frozen breath on a cold winter's day, says Schumann.

Not all aircraft produce contrails – they are estimated to occur in about 18% of flights. The air needs to be cool enough for the water to freeze, which is why they usually only appear above certain altitudes – typically 20,000ft (6km).

Even fewer flights produce the most persistent contrails. In clear, cloudless air, contrails disappear quickly, as the dry ambient air makes the ice particles sublimate (pass from a solid to a gas). But if the atmosphere is humid, the ice particles cannot sublimate and contrails can linger for far longer.

"Contrails may only last a few minutes if the surrounding air is dry, but when it's humid, they can persist and spread out to increase cirrus cloudiness," says Tim Johnson, director of the Aviation Environment Federation (AEF), a UK-based non-profit.

This matters because contrails and increased cloudiness trap heat radiated by the Earth, leading to a net warming effect on our planet, he adds. This warming is in addition to aviation’s other major climate impact – the CO2 spewed out in vast quantities from its tailpipe, contributing around 2.5% of global CO2 emissions

But contrails’ climate impact is far more complicated than that of these CO2 emissions.

In the 1960s, papers first started to appear noting that contrails could be having a cooling effect on the earth, says Schumann. This occurs because the whiteness of the induced cloudiness from contrails reflects solar light back away from the Earth’s surface during the daytime, in a similar way to natural cirrus clouds, which are also made of ice crystals.

However, scientists soon realised that contrails can also have a warming effect on the Earth, via a "greenhouse" effect catching infrared light. "They capture the heat which is emitted from the Earth's surface and emit part of it back to the surface," says Schumann. It's similar to the way cloudy skies produce warmer nights – the clouds above are capturing some of the outgoing heat, although contrails are not large enough to cause weather changes on the ground.

Depending on the exact conditions, one or other of these opposing effects can win out. For example, contrails which form over a snow-covered landscape don’t lead to additional cooling from solar light being reflected, because the surface is already white and reflective. Contrails also warm more at night because no solar light is arriving to be reflected by them, so only the warming effect is at play.

Along with the difficulty in understanding the exact atmospheric conditions where contrails form, and the trickiness of differentiating them from natural cirrus clouds once they spread out across the sky, all this makes setting clear numbers on the climate impact of contrails complicated.

However, the present understanding is that "on average over all countries all over the world, in over a year, contrails do warm", says Schumann. The latest major paper on the climate impacts of contrails, published in 2020, estimated that non-CO2 impacts of aviation – dominated by contrails – lead to a tripling of the "radiative forcing" of CO2 emissions alone.

In some ways it’s a tricky comparison, because the warming effects of contrails and CO2 occur on very different timelines. "Contrails have a very strong forcing effect, far stronger than the CO2. But contrails are short lived, they are gone after an hour or so. Whereas the CO2 lives very long, it can live for 100 years," says Schumann.

Still, scientists have warned that the heat-trapping effect of contrail clouds could triple by 2050 if no action is taken.

The good news is that this could actually be a fairly simple thing to tackle. Researchers have shown that just 2.2% of flights contribute 80% of this forcing, and that relatively small adjustments to the altitudes of these flights – at a small fuel cost – could hugely reduce contrails’ warming effect. Contrail formation can also be reduced by reducing the amount of soot particles emitted by flights, research has indicated, as these provide the nuclei for ice crystals to form.

Avoiding flying through very humid air, where persistent contrails can form, is the most important measure here, says Schumann, by flying above, below or around these regions. Better weather predictions are needed to facilitate this, however, he says. "The weather predictions which we have today are not accurate enough for that purpose."

In one letter to the journal Nature last year, two researchers argued that tweaking of flight altitudes to minimise contrail formation could be one of the most cost effective climate measures around. They calculated that preventing the most damaging contrails would cost $1bn (£850,000) per year, with a benefit worth over 1,000 times this amount. "We know of no comparable climate investment with a similarly high likelihood of success," they wrote.

However, while governments recognise the climate challenges posed by contrails, there has been little policy action to date, says Johnson. "The climate targets set for the sector by industry, countries, and the UN, all relate to CO2 emissions only. The debate around scientific uncertainty and appropriate metrics is often cited as a reason for focusing on more research rather than action."

In the UK, the Government's climate change advisers have said non-CO2 effects like those from contrails need to be brought into sharper focus. Considering these effects now is essential, especially in considering how future technologies such as hydrogen can reduce the sector’s emissions, says Johnson.

Despite the complications, the warming and cooling impacts of contrails are well-established science. However these wispy lines hovering above us in the sky have also inspired a theory that very much isn't backed up by science: that they are trails of toxic chemicals nefariously sprayed into the atmosphere by tens of thousands of commercial aircraft.

"People who believe in the chemtrails conspiracy believe that contrails are deliberately sprayed for evil purposes," says Amy Bruckman, a researcher in social computing at the Georgia Institute of Technology, who last year co-authored a paper looking at how believers in the chemtrails conspiracy had come to the belief. "They have different opinions on what those purposes are. Some people believe it to be population control, or mind control. [...] A number of people believe that their health problems are caused by the spray."

The chemtrails conspiracy is an astonishingly widespread belief. A 2016 poll found that 10% of Americans consider the chemtrails conspiracy to be "completely true", with a further 20-30% considering it "somewhat" true. The belief was not divided by party lines, and unlike some other conspiracy theories, chemtrails appear to be believed in by both men and women equally, says Bruckman.

Chemtrails trended on social media several times this summer in the UK, where fact-checking website Full Fact regularly finds itself debunking photos and other claims of or about supposed chemtrails.

The belief is often linked to concern that governments or companies are implementing covert, large-scale weather modification. Some 61% of English-language social media posts about geoengineering between 2008 and 2017 were linked to the chemtrails conspiracy. Solar geoengineering advocates have complained that the chemtrails conspiracy is so huge it has gotten in the way of real public deliberation about solar geoengineering.

The chemtrails conspiracy theory began in the 1990s, but burst into widespread belief in the early 2000s as it spread across the internet. "Suddenly in 2001 many people talked about chemtrails, it was a sudden appearance," says Schumann. He notes that the chemtrails conspiracy is "totally illogical" – his institute measures the emissions behind aircraft and has found no artificial chemical species. "It's all things which you would easily understand from the combustion of kerosene. There's no evidence at all that chemtrails exist."

This evidence is regularly dismissed by conspiracy theorists, however. One colleague of Schumann’s who cited his work to a chemtrail believer was promptly told Schumann was an "invented scientist" who doesn’t exist, he says.

It’s not hard to understand why for more than a century now, people have been pondering the effect of these slender, man-made lines in the sky. The real need now, though, is to focus on better understanding – and taking action on – their climate impacts.

Jocelyn Timperley is a senior journalist for BBC Future. Find her on Twitter @jloistf

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