The trail of clouds that billow from an airplane streaking across the sky can be mesmerizing for children and adults alike. Jet engine traffic has become so common that it's not unusual to see several lingering streaks in the afternoon skies. And though many think these cloudy bands are beautiful against a bright blue sky, others are alarmed by their mere existence. Some people with concerns believe these cloudy trails are exacerbating global warming, while others have more elaborate theories, including that governments are secretly releasing harmful substances into the air and land via airplanes [source: The Keith Group].
Before we get into the various theories about the possible harmful effects of these contrails, let's first discuss the scientific explanation for these trails. Jet engines spew out very hot air, and because water vapor is also one of the byproducts of the exhaust, the air is also very humid [source: Ackerman]. However, high in the atmosphere where these jets fly, the air is typically very cold — often below -40 degrees Fahrenheit (-40 degrees Celsius). Additionally, the atmosphere at that height has low vapor pressure, or the force exerted by a gas on the surrounding environment.
When a jet engine is emitting out hot, humid air into an atmosphere that is so cold and has low vapor pressure, the result is condensation. The water vapor coming out of the engine quickly condenses into water droplets and then crystallizes into ice. The ice crystals are the clouds that we see forming behind the engine. That is why the streaks we see are called contrails, short for "condensation trails." To help explain it, scientists liken it to seeing your breath on cold days. You may have noticed that puffs of breath dissipate quickly on drier days. The same is true of contrails: When the atmosphere is more humid, the contrails linger longer, but when the atmosphere is dry, the contrails disappear more quickly. Contrails can last for hours and cover great distances [source: FAA].
This explanation makes sense. But, as author and airline pilot Patrick Smith wrote, the contrails consist of not just ice crystals and water vapor, but also of other byproducts of engine exhaust, including carbon dioxide, nitrogen oxides, sulfate particles and soot [source: Smith]. Some point out that these, in addition to the extra cloud cover, can have negative environmental effects. And conspiracy theorists have nicknamed contrails "chemtrails" because they suspect that governments are taking advantage of this scientific phenomenon to secretly release other substances into the atmosphere.