You see them in the sky just about every day, especially when it's clear out.

Pat and Carl Schmidt, a local couple, reached out to CBS5 for answers, concerned these cloud-like trails coming from airplanes could be affecting their health and yours.

An Arizona State University professor I spoke with said there might actually be some cause for concern but not for the reasons you might think.

The Schmidts took video and photos of the long trails as seen from Surprise. If you live in the Valley, you've probably seen similar trails and have maybe wondered what they were and why there were so many.

"It lingers and it spreads and eventually after a while, it covers the sky," Carl Schmidt said. "We don't want to sound like conspiracy lunatics because we aren't into that. No, we aren't into this conspiracy stuff."

The idea of chemtrails gained traction in the mid-'90s and is popular among conspiracy theorists who believe the trails we see behind planes are made up of toxic chemicals or biological agents deliberately dispersed into the atmosphere. The reasons believers give for chemtrails are varied -- everything from manipulating the weather to military weapons testing to controlling the human population.

"We just want a straight answer as to what it's doing, why they're doing it, who's behind it and give us a straight answer," he continued.

My search for that "straight answer," led to Michael Line, an assistant professor at ASU's School of Earth and Space Exploration.

After showing Line the video the Schmidts sent us, he said there's no need to worry, adding what the Schmidts are seeing, are contrails -- not chemtrails -- and pose no risk to the public.

"So you can think of them as basically clouds that form behind the airplane," Line explained. "So what happens is, is the airplane exhaust puts out small amounts of soot particles -- just as a part of combustion, just like your car would -- and that serves as cloud condensation nuclei for which the water droplets or the water vapor can form water droplets, then freeze out onto, thus forming the cloud."

[NASA: The Contrail Education Project]

According to NASA, there are three types of contrails. Short-lived - short white lines following a plane Persistent non-spreading - white lines that remain visible after the plane is out of sight Persistent spreading - white lines that are long, broad and fuzzyNASA calls contrails, which is short for condensation trails, "'human-induced' clouds."

Some of those trails appear to form much lower in the atmosphere, which is where the Schmidts' initial concerns came from.

Line says the height of the contrails can be deceiving.

"They generally only form, say above 25,000 feet where it's really cold, say below minus-40 degrees Fahrenheit," he said. "So the ones that look like they're really low to the ground, they're actually really high, but it's just because they spread out because more water droplets have condensed out."

Line said you'll see fewer contrails on a day that's dry and clear because there isn't as much water vapor to form the droplets behind the airplane. It's on humid days that you will tend to see more of the trails.

Line says contrails can play into global warming because they tend to trap the heat in Earth's atmosphere.

With hundreds of thousands of flights per day, there's a large uncertainty as to how big of a role they play, but Line says they can play a small role.

Copyright 2017 KPHO/KTVK (KPHO Broadcasting Corporation). All rights reserved.

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