Why are there white lines in Houston skies? The conspiracy theory behind them

Looking at the sky on any clear or even partially cloudy day, you might see an outline of an aircraft with what seems like white steam trailing behind it. Most people assume that trail is some form of condensation and they go about their day. 

For almost 30 years, many have seen them as “chemtrails” and think it’s part of a secret mind-control conspiracy. Or a plot to sterilize humanity. Or an attempt by the government to alter the atmosphere.

The chemtrail conspiracy theory has spread so much since 1996, that it’s one of the most widely believed conspiracies in the world.

The conspiracy even prompted the Tennessee legislature to pass a bill, which the state’s governor signed last week, that “prohibits the intentional injection, release, or dispersion, by any means, of chemicals, chemical compounds, substances, or apparatus within the border of this state into the atmosphere with the express purpose of affecting temperature, weather or the intensity of the sunlight.”

The reality is that the white lines of vapor are called contrails, and even though they affect the environment, scientists have found no evidence of them being part of a global conspiracy — yet that hasn’t stopped the conspiracies from spreading. 

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The Tennessee legislature had introduced the bill to prevent “geoengineering” from being tested in the state. Geoengineering is a controversial area of climate policy because, if tested on a larger scale, it could involve physical risks and raise a range of sociopolitical and ethical issues, according to the David Keith Research Group at Harvard University. Still, geoengineering research is new and there’s no evidence of any large-scale geoengineering programs.

The Tennessee lawmakers mentioned the chemtrail conspiracy when debating the bill, which made it clear that lawmakers weren’t just targeting geoengineering testing. Tennessee isn’t the only state to introduce this type of legislation: Lawmakers in seven other states also have attempted to enact anti-geoengineering policies.

“This idea of chemtrails as a conspiracy theory has always just sort of been off to the side lingering in the background as sort of noise,” said Richard Moore, a research physical scientist at the NASA Langley Research Center.

Moore studies the emissions that come from aircraft engines and their effect on the environment. He said most of what is emitted from aircraft engines is carbon dioxide and water vapor with a small amount as unburned combustion products like soot particles, nitrogen oxides and sulfur dioxide.

No credible evidence shows contrails are a deliberate attempt to alter human behavior or the environment, Moore said. But contrails are human-made products that can alter the environment to a certain degree.

“Just like the tailpipes that we see from our cars and trucks that emit exhaust, aircraft produce a visible cloudiness that is human-produced,” Moore said. “So, humans are affecting the environment. We are affecting the climate through aviation and the formation of contrails. But the idea that this is some deliberate attempt to form that cloud is just not true.”

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It’s common to see white smoke coming from a car’s exhaust pipe on a cold day. The water vapor caused by the engine’s combustion process is very hot and when it mixes with cold air, the water vapor wants to condense from a gas to a liquid. 

On the Earth’s surface, the air is normally too warm to sustain a cloud so it rapidly evaporates a few feet behind the vehicle. In the atmosphere, the air is much colder, so the water vapor from an aircraft, combined with the soot particles that are emitted from the engine, can form a wispy cirrus cloud.

“When we start thinking about this cloudiness that humans have created by flying our airplanes through the air, these cirrus clouds are going to interact with the sunlight coming into the Earth, and also the heat that comes up from the Earth’s surface,” Moore said. 

Moore studies the cooling effects contrails create by reflecting sunlight back into space and the warming effect associated with trapping outgoing radiation from the Earth’s surface. Moore said the warming effects make a larger difference globally, particularly at night.

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“If you add up the effect of all the contrail cirrus clouds over the whole globe, it’s actually a pretty significant climate impact,” Moore said. “We think the magnitude of that impact is of the same or even greater than that from the emissions of aviation carbon dioxide.”

According to a 2022 report from the U.N. Intergovernmental Panel on Climate Change, contrails may be responsible for most of the airline industry’s effect on climate warming. 

Researchers and airlines are looking into ways to curb those airline emissions, like flying at different altitudes to avoid creating contrails. Although it’s still difficult to predict which contrails have the greatest effect on climate, airlines would still need to balance avoiding contrails and burning more fuel to reroute a plane, according to a report by the Washington Post. 

Other possible solutions include creating low-emission aircraft engines and moving away from petroleum-based jet fuels. Although the engines and fuels wouldn’t eliminate contrails, they could change the properties of the contrails so they have less effect on the climate, Moore said.

According to Moore, NASA’s approach to conspiracy theories is to ignore them.

“We try to steer clear of the conspiracy,” Moore said. “We focus on the science and the research to benefit the nation and society. We’re here as scientists and active members of the science community to educate the public and tell them about what we’re doing and how that may impact the climate and their lives.”

Moore said conspiracy theories can cloud discussions about how contrails affect the environment and can prevent real, policy-relevant discussions.

“We’re very fortunate that these larger societal or policy debates motivate our work, but they don’t interfere,” Moore said.