DENVER (NBC NEWS)- They look like chalk marks across a giant blue blackboard, a criscross pattern of white vapor in the skies. Some call them chemtrails short for chemical trails.

Scott Stevens says "I had viewers and readers writing me that something was not right," said Scott Stevens, a former TV meteorologist from Idaho.

He now lives in Fort Collins, and studying contrails has become his passion.

"I put cameras outside of my house in Idaho, and it was about four months after doing that, that I found one particular sortie of flights in the sky where the planes would leave a trail and what was curious was that other planes would come along and hit precisely over the debris or what was left over of the previous trail remnants," Stevens said.

Observers like Scott believe that government agencies are trying to modify the weather with these contrails.

"I began to notice other meteorological phonemena, cirrus bursts, openings in storms," Stevens said.

Professor Bob Mock says "in my first life I was a weather forecaster before I became a fighter pilot."

Bob Mock is a retired Air Force Colonel and is a professor of aerospace sciences at Metropolitan State College in Denver. He says that there's not really much to a contrail.

"A contrail is basically water that has frozen and formed a cloud behind the exhaust from a jet engine."

And that making a contrail is just the simple chemistry of combustion.

Bob Mock says "if we burn seven pounds of fuel, we manufacture ten pounds of water."

Professor mock is plenty well aware that contrails are everywhere.

Bob Mock says "there's one behind me as we speak interestingly enough."

But there's no reason to be alarmed.

"First of all there's a lot more airplanes and you're gonna see more in America simply because the United States has half of all the commercial flying in the world," Mock said.

And as for why there are some days we see more contrails than others, Mock says, "It's just a matter of the atmosphere and the conditions there are conducive to having that contrail sustain itself"

Professor Mock believes we can all breathe easy.

"I really don't think the government is doing anything aloft."

Bob Mock says that contrails of dry ice have been part of weather modification experiments in past years, but trying to make it rain is extremely difficult, at best.

Scott Stevens has a Web site dedicated to his theories about contrails. The site is listed in "Related Content" at the top of this story.