Why Do Jet Planes Leave a White Trail Behind?

When you look up at the sky on any given day of the year, you may spy the majestic sight of a jet zooming far above. If you’ve seen one of those planes speeding across the sky, then you’ve almost certainly seen those long white trails streaming out behind them!

Photo Credit: puchan / Shutterstock

Photo Credit: puchan / Shutterstock

As children (and as adults), we are often mesmerized by the patterns they can leave among the clouds, but they have also inspired concern in some people, and even some urban legends about government conspiracies!

Before you start panicking every time you see a white streak in the sky, however, let’s dig a bit deeper into the facts of this phenomenon of aviation.

The Science of Contrails

Those “white streaks” do have a scientific name – contrails, and there actually isn’t anything particularly mysterious about them. Contrail is a shortened version of “condensation trail”, which is precisely what all of that white smoke is! Flying a plane requires a lot of fuel, which creates a lot of exhaust, just like what happens when you drive a car. Mixed in with that exhaust is water vapor, just like the water vapor in our own exhaled breath.

Now, on a cold day, when we exhale, we can see our puff of breath, because the water vapor in our breath is solidifying into ice crystals before evaporating, or turning into water molecules. The exact same principle applies for jet exhaust. When planes fly above 25,000 feet or so, the temperature is very cold (approximately -30 degrees Fahrenheit). When those exhaust fumes are shot out of the back of an airplane, the water vapor hits the cold air and condenses, leaving the contrail behind. Depending on the humidity, temperature, and the chemical composition of the exhaust, contrails last in the sky for varying amounts of time, and those variables also determine the size and length of the trails.

These contrails are more than pretty streaks, however; they can also be used to determine changing weather patterns or oncoming storms – if you know how to read them! On a slightly more concerning note, some researchers and scientists have studied the environmental impacts of contrails, particularly their impact on the greenhouse effect. Having large amounts of ice crystals in the blanket of molecules, moisture, and gases in the upper atmosphere can contribute to greater trapping of greenhouse gases, and continually higher temperatures on the planet.

Photo Credit: LSkywalker

Photo Credit: LSkywalker

So, next time you’re gazing up in the sky, watching the patterns emerge as planes criss-cross far above, you’ll know exactly why you’re seeing that long lazy trail…. or will you?

References:

NASA

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