Contrails stretch above Telluride along the San Juan Skyway Scenic Byway outside Telluride, Colo, in October 2016. (Bonnie Jo Mount/The Washington Post)

It’s tough to go outside nowadays without seeing scores of lines in the skies. They trail behind airplanes, forming by the dozen in regimes of heavy air traffic. Some spread out into a wispy veil that blankets the sky, whereas others linger for what seems like a very long time.

Controversy has prompted speculation that these aircraft-chasing clouds may be a harmful mixture of chemicals sprayed to influence humans behavior and to modify the weather, sometimes referred to as “chemtrails.” But is there any truth to this theory?

Nope. Not a bit. Period. No need to don a tinfoil hat!

“Contrails,” as the lines are aptly called, are a man-made cloud that forms under otherwise ordinary environmental conditions. The underlying physical processes are the same as in cases involving any other clouds; the only difference is what mechanism starts them forming.

High up in the atmosphere, where aircraft fly, temperatures can fall well below freezing even on a warm summer’s day. Generally, there is little water vapor present this far aloft, since air’s ability to hold moisture decreases as the temperature drops. Moreover, water vapor residing there is mostly “supercooled” — meaning that it remains in the gas or liquid state despite temperatures colder than Antarctica. Why does this moisture not immediately become ice? The answer lies in a process called nucleation.


This NASA image, captured on board the International Space Station in April 2013, shows contrails from jet traffic over San Francisco. (Chris Hadfield/NASA/AFP)

For water to form ice crystals, it has to have something to freeze onto. This can be a piece of dust, pollen, ash or any other particulate floating around in the upper troposphere. The trouble is that it’s really tough to mix ground-level matter all the way up there. But airplanes can serve that function.

Commercial jetliners fly higher than the peak of Mount Everest, which towers to a whopping 29,035 feet. Their exhaust can spit out aerosols, sulfates, soot, minute amounts of metal, carbon dioxide, hydrocarbons and a few other ingredients. It may sound scary, but it’s no cause for alarm; jet fuel is composed mainly of kerosene, which is also a common lighting fluid for household heating systems.

When an airplane races by, the particles in its wake can serve as the nuclei for supercooled water droplets/vapor to condense and freeze upon. Because temperatures can drop below minus-40, it doesn’t take long for a cascade of water molecules to latch on and join the process. Trillions of cloud droplets, about a hundredth of a millimeter across, are formed in a stream behind the airplane. Just like that, a contrail is born.


Ducks fly beneath contrails in the sky at dawn in west London in November 2014. (Toby Melville/Reuters)

But that’s not the only thing going on there. We have to remember how airplanes fly! By manipulating the shape of aircraft wings and relying on something called the Bernoulli Principle, engineers have designed aircraft such that they slice through the air and induce a pressure gradient. In other words, the rush of wind generates a pillow of high pressure beneath the plane to suspend it in the air, while low pressure above spawns a miniature “vacuum” and makes it easier for the plane to rise. This “wingtip vortex” of low pressure is ordinarily invisible — but not always.

When air expands in regions of low pressure, it cools, often falling to the dew point and becoming saturated. This will make a visible cloud if the air is moist enough. That’s why contrails don’t have to form behind the engines of airplanes, since the wingtip vortices spin up off the outer edges. If you’ve ever seen a plane take off in fog or rainy weather, you may have noticed little tubes of cloud whirling behind; now you know why.

Some have pointed at what they consider to be “unusual behavior” associated with contrails, making disjointed attempts to link them to weather modification or toxic releases, but they are not supported by the mainstream scientific community and peer-reviewed studies.

To be sure, not all contrails behave the same way. Some spread out, blow away, or quickly disappear, while others remain in place. Straight forward science explains this activity.


Contrails from jet planes passing overhead intersect the National Museum of Art in Washington in April 2014. (J. Scott Applewhite/AP)

Even on a perfectly calm day, the winds way up there are howling. Airplanes fly at the level of the jet stream. On days where the jet stream is above you, you’ll probably notice the contrail clouds quickly streaming off to the east. Yet if the upper-level winds area weak, they may stick around, until eventually dissipating when molecular diffusion takes place. How long they’re visible for is a function of humidity levels as well.

Some claim contrails are the primary instigator of climate change. This is not supported. Even if contrails spread out across the sky, their thin nature is transparent to most incoming and outgoing radiation. While the shadows cast by them has a net cooling effect, it’s negligible — only a fifth of a watt per meter squared less of incoming solar rays. This is the equivalent of trying to warm the inside of your car with a single bulb from a strand of Christmas lights.

Other folks have stated that something about contrails isn’t quite right since they may appear different in color than “normal” clouds. Contrails are in every way, shape, and form, a normal cloud — just a man-made one!


The contrails of two airplanes fade in the sky as the rising sun adds a spectacular color cast to the landscape east of Budapest in January 2015. (Zsolt Czegledi/European Pressphoto Agency)

As for their color, we must remember where these clouds live — 37,000 feet or higher. The sun sets later at these altitudes because of Earth’s curvature. While shallow clouds near the ground may appear darkened due to the loss of daylight, contrails and high cirrus clouds can still soak up the sun and glimmer an amber shade for 10 minutes after nightfall. They can be a beautiful sight to behold. This is the same reason that the tips of skyscrapers in the city can be bright even after the sun has set on ground-dwellers. At the world’s tallest building, the Burj Khalifa in Dubai, the sun sets on the 160th floor a full five minutes after the lobby.

The science behind contrails is fascinating. Contrails should never be a cause for alarm; after all, folks don’t flip out on chilly days when their breath forms a cloud. If it’s cold enough and the air is still, you might even notice a cloud hanging behind you for several meters. Contrails are a unique form of cirrus clouds, and can be an early indicator of approaching weather systems. Conspiracy theories aside, learning the underlying physics supporting their formation can broaden our understanding of the world around us.

The atmosphere puts on beautiful shows for us every day. we just need to know where to look.

(Clarification: The original version of this article stated contrails form from commercial airliners; as they also form from military and private aircraft, the describing language was amended.)

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