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.