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Is there something sinister in airplane contrails? See more flight pictures.

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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. And though many consider the streaks beautiful against a bright blue sky, others are alarmed about them. Concerns range from the idea that these streaks could exacerbate global warming to more elaborate theories that the government has secretly been dumping harmful substances on the land.

Before we get into the various theories about the possible harmful effects, let's discuss the scientific explanation for these streaks. Jet engines spew out very hot air. And, because water vapor is one of the byproducts of the exhaust, the air is also very humid. However, high in the atmosphere where these jets fly, the air is typically very cold -- often lower than -40 degrees Fahrenheit. Additionally, the atmosphere up there is often of low vapor pressure, or the force exerted by a gas on the surrounding environment.

When a jet engine is spewing out hot, humid air into an atmosphere that is 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 form behind the engine. This is why the streaks 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 dryer days. The same is true of contrails: When the atmosphere is more humid, the contrails linger, but when the atmosphere is dry, the contrails disappear more quickly.

This explanation makes sense. But, as author and airline pilot Patrick Smith tells readers, the contrails consist of not just ice crystals and water vapor but also other byproducts of engine exhaust. These include 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" under the suspicion that the government is taking advantage of this scientific phenomenon to secretly release other substances into the atmosphere.