Look! Up in the sky! It’s a bird … it’s a plane … it’s a … vapor trail. You know what a vapor trail is, although you may have never given it much thought. Vapor trails are a byproduct of aviation. They’re ubiquitous, yet often unnoticed. A vapor trail is also called a condensation trail, or contrail for short. It’s a skinny line of cloud that is left behind by a high-flying aircraft. Vapor trails came into existence a century ago as high-flying aircraft became more common.
Whether it is a propeller plane or a jet plane, they all burn fuel. Most of what the combustion of petroleum creates is carbon dioxide, but there are other compounds. The one that plays into vapor trails is water vapor. You’ve probably seen water droplets come out of the tailpipe of the vehicle in front of you. When planes burn fuel at high altitudes, where the temperature is very low, the water vapor instantly condenses to form a cloud. Technically, the water vapor is changing directly to ice crystals in a process called deposition. That’s exactly what happens in your freezer, but it’s a lot slower in there.
The ice crystals that trail behind an airplane are a cloud, not smoke. How long the trail lasts depends on how moist the air is at flight level, typically well above 25,000 feet. If air is really dry, there is no contrail. If air is moderately humid, you may see a short or medium contrail, stretching out a few miles. When air is very humid, condensation trails can stretch out dozens or hundreds of miles. The longer they last, the more they spread into unique and sometimes odd shapes. You can see ripples, sheets, rows, curves and wisps — all cirrus clouds that started as vapor trails. Those can lead to bright areas and colorful arcs in the clouds, due to the bending of light in the ice crystals. Because they are many miles above the ground, vapor trails will light up first, approaching sunrise, and they will darken last, after sunset, often with yellow, orange or red tints.
On the Gulf Coast, we are under a major east-west flight corridor, making condensation trails common. We see more than ever because air traffic is higher than it ever was. There are many studies underway to figure out how the extra clouds created by airplanes might impact Earth’s temperature balance, as they block a little of the incoming solar radiation. They don’t block enough to stop the overall planet warming.
Now, for a quarter-century, there has been a conspiracy theory circulating that condensation trails are an attempt to harm or control humanity by releasing chemicals in the air as “chemtrails.” There has never been any evidence or logic behind this. Condensation trails are easily explained by science.
Alan Sealls is chief meteorologist at NBC15, and an adjunct meteorology professor at the University of South Alabama.
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