Why airplanes leave white streaks in the sky

If you’ve ever looked up at a mostly blue sky and seen straight white lines criss-crossing the horizon, or watched a plane puff out a plume as it passed above, you may have wondered, “what causes that?” Condensation trails, or contrails for short, are not simply exhaust, like you might see from a car tailpipe (though they’re similar). And no, they’re also definitely not “chemtrails.” 

What they are, instead, is frozen water vapor crystallized on soot particles, both of which are standard byproducts of a jet’s combustion engine. In brief “a contrail is an artificial cloud,” says Stephen Barrett, an engineering professor at Cambridge University who studies the environmental impacts of aviation. “They’re very much the same as natural cirrus clouds, except they’re initially long and straight,” he tells Popular Science. And though conspiracy theories about airplanes distributing mind control chemicals aren’t correct, contrails are having a negative planetary effect. 

How do contrails form?

Despite being as simple as water vapor and dust, contrails don’t always form or linger in a plane’s wake. Atmospheric conditions have to be just right to enable the jet vapor to crystallize: moist enough that the water doesn’t evaporate, and cool enough that it freezes. “Air that’s cold enough or humid enough is called ice supersaturated,” says Barrett. 

Compared with other atmospheric elevations, ice supersaturation is relatively common at standard commercial jet cruising altitude (about 35,000 feet up). Yet still, it only happens about five to 10 percent of the time, he adds. 

The length of time a contrail lasts, or its “persistence” is also dependent on conditions. Warmer and drier air often means they’ll dissipate in a matter of seconds or minutes–but the right recipe of cold and wet can leave contrail clouds sticking around for up to six hours, spreading wide across the sky, Barret explains. 

Because contrails are so closely tied with temperature and air moisture content, they vary by season and region, says Marc Stettler, a professor of transport and environment at Imperial College London. In many places, they’re a more common sight during the spring and fall, Stettler notes. 

Are contrails dangerous?

Sort of–but not in any direct way to human health, and probably not how you’d expect. A growing body of scientific research, including studies conducted by both Barrett and Stettler, indicate that contrails cumulatively have a warming effect, contributing to human-caused climate change. 

Of course air travel contributes to climate change because planes burn fossil fuels to fly and add carbon dioxide into the air. But the thermal effect of contrails is a separate phenomenon, distinct from aviation’s greenhouse gas emissions. And it’s a surprisingly potent impact. 

“The contrails from aviation cause about as much warming as all the CO2 from aviation,” says Barrett–with one key difference. The carbon emissions from flying are cumulative, adding up over the course of decades, whereas contrails are temporary–dissipating after only a few hours. “So that means the last six hours of contrails cause about as much warming as the last 60 years of [aviation] CO2. It’s quite a strong lever,” he says. 

In other terms, Stettler explains that contrails make up an estimated two percent of all the human-caused warming in any given year–that’s including everything, from car emissions to agriculture to heating our homes. Two percent may not seem like much, but some sources estimate commercial air traffic is set to multiply by 2.5 times over the next 30 years, and when Earth is on the line, everything counts.  

How do contrails warm the climate?

Since contrails, like other clouds, are bright white and insulating, they reflect light and heat, and also trap it. On a sunny day, a contrail does two things simultaneously. It “acts like a blanket,” preventing heat radiating from Earth’s surface from escaping to space, says Barrett–this is the warming effect. Simultaneously, it also reflects sunlight from space away from Earth’s surface, in a cooling effect. Unfortunately, even when the sun is shining, generally the blanket effect outweighs the reflector effect, says Barrett.

And when the sun sets, there’s no contest. During nighttime, there’s no space rays for contrails to reflect, only heat for them to keep trapped. Though contrails are impermanent, the heat that they bounce back to our planet’s surface doesn’t miraculously make an escape once the initial impediment is gone. Plus, so many airplanes are in the sky at any one time that if conditions are right, there’s probably already another contrail already there to replace whatever’s dissipated. 

How can we solve it?

Ice supersaturated regions can be very widespread bands across the atmosphere, but they’re often very thin, says Barrett–which means pilots can avoid them by flying slightly above or below where conditions are right for contrails. In many cases, it might only take an adjustment of 1,000 or so feet, “pretty small relative to cruise altitudes,” he adds. In other words, very slightly altering flight routes could make a big difference. 

Currently, we’re not great at forecasting atmospheric conditions at 35,000 feet, says Stettler. “There isn’t a huge amount of data for it, and it hasn’t been a major priority for meteorologists,” he explains. But researchers are working to improve our ability to predict atmospheric conditions and to plan flights accordingly. In the interim, simply responding to conditions in real time as pilots currently do to minimize turbulence, and adjusting altitude as soon as a contrail becomes apparent might significantly reduce the global impact, Barrett says.   

Already, there’ve been small scale trials for forecasting efforts, including one involving Google, Breakthrough Energy, and multiple airlines. But more large tests are needed to better understand how to best mitigate contrails, Barrett notes. “That’s really the next challenge–let’s say all of Scottish airspace or a big chunk of the Atlantic tracks–that’s where we need to go next.” 

Yet even if we get contrails under control, the aviation industry still needs to do more, notes Stettler. “Reducing the effects of contrails should not replace or substitute action to mitigate aviation’s CO2 emissions,” he says. After all, if all the contrails disappear in one final puff, there’d still be decades of lingering emissions to worry about.

This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.