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Why Planes Leave White Streaks In The Sky

by SharonKurheg

Have you every noticed, especially on a clear, sunny day, that planes leave a trail of what looks like white smoke in their wake? Ever wonder why? I did. Here’s why it happens…

It turns out it’s all due to condensation. A plane’s exhaust gases have moisture in them, and when the plane is at higher altitudes (think 20,000 feet and higher), the colder temperature and along with the low air pressure forces the moisture to condense. The condensed moisture vapor freezes if it’s cold enough, and when it does, this makes something that looks like thin, white clouds that trail the plane’s engines.

The trails are commonly known as contrails (short for condensation trails) or vapor trails. It’s sort of like what happens when you can see your breath on a really cold day – the moisture in your breath meets the cold air and freezes.

Sometimes the vapor trails are thin and sometimes they’re fluffy. Sometimes they’re inconsistent and eventually start or stop. All of that is due to the plane’s altitude, as well as the outdoor temperature and humidity of where it is at the time. So if you see two planes at the same time but they have two totally different shapes of trails, or if one has a trail and one doesn’t, it’s because they’re at different altitudes, and therefore different air temperatures and air pressures.

Fun Fact! What the trails look like can help predict the weather. For example:

  • Fair weather = a thin, short-lived contrail (it usually means there’s low humidity in the air at that higher altitude)
  • Storm is coming = a thick, long-lasting contrail (more humid air at higher altitudes)

There’s a conspiracy theory that the trails are made of chemical or biological agents that are being sprayed onto the public for evil purposes (they call it “chemtrails”), but….yeah, no. Contrails do contain pollutants from the planes’ exhaust, but not chemicals that are purposely being sprayed on us for who knows what nefarious reasons.

Speaking of chemicals…contrails are not the same as what skywriters use. Skywriters use smaller airplanes that have special smoke machines to make their written messages you can see from the ground. The smoke machines usually use pressurized containers that have oil in them. At a pilot’s command, the machines spray the oil onto the plane’s exhaust system. The exhaust system is hot so the oil burns quickly and creates clouds of dense white smoke.

Feature Photo: Max Pixel

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This post first appeared on Your Mileage May Vary

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