Have you ever watched an airplane fly across a clear blue sky? Sometimes it’s easy to trace an airplane’s path, because it leaves behind a white streak across the sky. After a while, though, the plane’s track fades and disappears into thin air.

Those white streaks planes leave behind are actually artificial clouds. They’re called contrails, which is a shortened version of the phrase “condensation trail.”

Airplane engines produce exhaust, just like car engines do. As hot exhaust gases escape from a plane, the water vapor in the fumes hits the air. At heights of 26,000 feet or more, the air is extremely cold (sometimes more than -40° F!).

The cold air causes the water vapor to condense. This means the water vapor gases turn into tiny water droplets or even freeze into tiny ice crystals before eventually evaporating. This condensed water vapor and mixture of ice crystals make up the cloud-like trails you see in the sky.

If you’ve ever seen an air show, you may have seen airplanes creating messages in the sky with what look like clouds. Are these contrails? Not exactly…

Skywriters use small airplanes equipped with special smoke machines to fly in special patterns to create written messages visible from the ground. The smoke machines usually consist of pressurized containers full of oil. At a pilot’s command, the machines spray oil onto the plane’s hot exhaust system, where it burns quickly and creates clouds of dense white smoke.

Pilots have to learn how to fly special patterns and work the smoke machine carefully to be able to create their unique messages in the sky. From advertisements to marriage proposals, the messages can be practical, personal or just plain silly.

Skywriting goes way back. There are reports of successful skywriters before World War I, possibly as early as 1915. Today, skywriters take advantage of satellite navigation to program messages before flight, thereby increasing accuracy.

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    • Hi there, Tera, thanks for joining us at Wonderopolis today! We sure are glad that you commented on our flying Wonder, too! Those artificial clouds in the sky are actually exhaust gases from the airplane. You might have seen exhaust gases from the tailpipe of a car– the same is true for airplanes! We hope you have a great day, Tera! Please come back and visit us soon! :)

  1. Tera’s right… They are spraying! If there are trails that stay long in the air, I don’t know what that is… But, if it disappears, than it’s normal cloud. Other “stories” are fake.

    • Hi EH! Thanks for WONDERing with us! Planes leave the lines behind them as the exhaust leaves the engine and combines with the air vapors! Thanks for WONDERing with us! :)

    • Hi Rich! Thanks for WONDERing with us! It could be used for communication purposes! Some airplanes even write words in the sky with their tracks! Have you ever seen that before? Thanks for WONDERing with us! :)

  2. If that is true, why don’t I have to drive back through my exhaust a few hours later if I am returning from where I just drove, or see auto exhaust fumes that linger all day long next to roadways? Planes leaves these contrails in the skies all day sometimes and change the sky to a nasty white haze. It just doesn’t add up to harmless water vapor. More like geoengineering. No matter how you look at it, there is too much of this going on in our atmosphere, and that cannot be great for the environment, or the air we breathe, because what goes up, must also fall down.

    • Excellent argument, Laney! Keep in mind the reason contrails linger in the sky so long has a lot to do with the temperature at that height. Thanks for WONDERing with us, today! :-)

  3. Here’s the thing, I am over 70 years old and I love a blue, blue sky or large beautiful clouds and have always gazed at the sky everytime I am outside (almost always). These so called chemtrails are something new and I do not think it’s normal procedure for a jet to fly back and forth (or several planes) until the sky is covered with these trails Whatever it is, you can bet it’s not normal.

    • Thanks for sharing your thoughts, Jan! We think it’s WONDERful that you spend so much time outside! We think you’re right, that it’s not normal procedure to fly back and forth and completely cover the sky with contrails — usually planes are just passing through, getting from point A to point B! :)

  4. But some time we seen two same that type of airplane in the sky but one airplane leave track for few minute and another airplane leave for more time…….?

    • That’s true, Mohit Dagar. Why do you think that is? We are glad you shared your thoughts on this WONDER. Have a WONDERful day!

  5. Those “exhaust trails” as you call it are nothing more the a toxic chemical concoction that they are spraying out to manufacture weather. In commercial flights, it’s a mix they’ve developed for jet fuel. Also, military often sends planes and drones to do mass spraying. The spray contains toxic reflective metals such as barium, lithium, strontium, boron, and much more. Which is why people are starting to see such strange colors at sunrise and sunset and some clouds during the day that appear pink, yellow, blue, and green. The also spray nano tech such as smart dust.

    • Hi Amy! Thanks for sharing your views on this WONDER. Some scientists are concerned that the exhaust trails may affect Earth’s climate. There are many different beliefs about this topic. However, this Wonder of the Day is simply a scientific explanation of how the exhaust trails are formed. We are sorry you did not agree with this WONDER. There are LOTS of WONDERS on WONDERopolis and we hope you will explore other WONDERS too! :)

  6. How does this tie in to Solar Radiation Management? My friends told me that they spray aluminum particles into the air in order to reduce global warming. How can you tell the difference in the normal contrails and the aluminum contrails?

    • Those are great questions, Joe! Maybe you could do further research on the topic at your library. Thanks for WONDERing with us! :)

  7. There are pictures all over the Internet of these trails that look like they have started and stopped. This is because the pilots have the ability to turn this aerosol on and off.

  8. Let me begin by informing you that I work in the Aerospace Industry as an engineer.

    Your article has some useful information. I do however believe that overlooked a CRUCIAL detail,
    This is completely wrong. You are buying into the narrative. I’m not arguing that planes exhaust water vapor as a result of combustion, nor am I arguing that the water condensation will dissipate and fade away. There is science behind what you are saying, but I want you to do something.

    Look at our sky. Next time you are walking around the yard or into wal-mart or whatever and you see the plane actually leaving the so called “condensation trails”, look around. It is almost guaranteed that you will not only see one. These planes are spraying whatever they are spraying in a strategical pattern. Return to that same spot each hour over the course of 3-4 hours. These trails do not dissipate. These trails do not fade until many many hours after they should have. The trails actually slowly broaden and BLANKET the entire sky.

    We have always just accepted this as “overcast” conditions.

    We have always just accepted the water vapor emission and condensation is what is being emitted from these planes.

    We as society do not pay attention anymore. And our government has definitely capitalized on this fact.

    You are more than welcome to disagree with me and my opinion, but seriously, WATCH YOUR SKY.

    It was only recently that someone told me to do exactly what I am telling you and I now see it. With my own eyes.

    Best of luck, Cheers!

    • Hi Ryan! Thanks for sharing your opinions about this WONDER! We acknowledge that there are many different beliefs about this topic. However, this Wonder of the Day is simply a scientific explanation of how the exhaust trails are formed. Best of luck to you, too! :)

      • I think somehow the first part of my post didn’t post properly. There was supposed to be more after “.. crucial detail” and before “This is completely wrong”

        Sorry if I came off as attacking, that wasn’t my intention, yet reading how it posted, it sounds mean! I’m sorry!! I was simply pointing out that these trails do not “fade and disappear” like your article said.

        This was my first visit to Wonderopolis and I will be returning. Thank you for your good sport and positive attitude when everyone is seemingly picking apart your article.

        Thanks again WONDERopolis!


        • Welcome back, Ryan! We appreciate you visiting WONDERopolis and sharing your thoughts about the WONDERS. We respect everyone’s viewpoint and want all our WONDER friends to feel comfortable sharing their thoughts on WONDERopolis. We also want to make sure that we are representing the scientific evidence behind this WONDER, which is how the exhaust trails are formed. This is why we chose to remove part of your posting. Thanks for commenting on this WONDER and we hope you will continue WONDERing with us on WONDERopolis. Check back for more WONDERful WONDERS! :)

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Have you ever wondered…

  • Why do airplanes leave tracks in the sky?
  • What are contrails?
  • How do pilots use planes to write in the sky?

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Try It Out

Ready to make your own contrails? OK! Grab the keys to your family’s airplane and head on up into the sky! What do you mean your family doesn’t have an airplane? Well, now that we think about it…even if they did have an airplane, they probably wouldn’t let you fly it just yet! Don’t worry, though, you can still have fun checking out one or more of the following fun activities with a friend or family member:

  • Do you ever make homemade cards for your friends and family members on their birthdays? There’s nothing quite like a heartfelt message written by hand in a card you made yourself. Of course, paper limits you a bit in terms of the impact your message can have. But what if you had the whole sky as your canvas? What would you skywrite to a friend or family member if you had the chance to write any message in the sky for them to see? Think about it and then tell that person what you would write if you could write in the sky. You never know when you just might make someone’s day!
  • Is it cloudy outside today? We hope so! Why? Because this activity requires some clouds. Head outside with some friends or family members and lay out a blanket so you can comfortably gaze at the clouds as they cross the sky. What shapes do you see? Pick out a certain area of the sky and see what you can see. Do your friends or family members see the same things you do? Compare your observations. How are they similar? How are they different? Have fun cloudgazing and using your imagination to give the random clouds some meaning!
  • If you’re up for a challenge, try this fun experiment at home with just a few simple supplies. Grab a clear plastic 2-liter bottle, matches (you’ll need an adult to help with this experiment) and some warm water. Just follow the directions to create your very own Cloud in a Bottle!

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Visit Illuminations’ Long Distance Airplanes lesson to make paper airplanes and explore attributes related to increasing flight distances.


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