Posted March 10, 2009 - As we all know, a jet doesn't leave a white trail when it takes off. Nor does one appear when it's low. But when it climbs high into very cold air the story changes.

Jets burn kerosene. If you've ever used a kerosene heater, you've smelled exhaust products rising from the unit. These include water vapor, bits of carbon soot, some carbon compounds like methane, and trace amounts of impurities like metals. Well, when jets climb above 30,000 feet, the air is so cold at -40° that the exhaust's water vapor instantly freezes into ice crystals. This is a contrail, a short white line behind the jet's engines. It often dissipates in seconds as each crystal sublimates back to invisible vapor.

Jets even higher up zoom through such cold air, it takes a long time for the contrail ice to re-vaporize. Higher humidity can preserve it, too. The contrail lingers as a long white line across the entire sky. Winds often distort these into various curves. If the air mass is very humid, the exhaust's soot particles enter the picture to act as condensation nuclei — like cloud seeding — to produce a thicker contrail as surrounding atmospheric vapor freezes as well.

In seconds the dense ice crystal formation spreads out and widens. It can expand into a long cloud that casts an odd shadow. Or, very rarely, in very high humidity, the contrail can act as the "starter yogurt" for a cloud covering the entire sky. People into meteorology have observed all these various contrail varieties since the 1930s. Contrail science is even taught in college courses, to show how an individual can assess the temperature and humidity at high altitudes.

These man-made pencil clouds reflect sunlight into space. Just a few years ago scientists were concerned that all the jet traffic would have a cooling effect on the planet. But global warming has made that issue go away. If anything, jet contrails are now seen as helpful.

Some folks regard contrails suspiciously. Apparently, many don't know what they are. Several websites call the lines chemtrails, and think that the US military is deliberately spraying a substance upon the population.

This is silly for a number of reasons. First, if you've ever watched crop dusting you know that chemicals must be released very close to the ground. Released on high, they'd dissipate with the wind and take forever to get down; the concentration on the folks below would be zero. Second, my commercial pilot friends (along with the controllers at the FAA) would all have to go along with the plot, since they'd see the process happening. I'm a pilot and airplane owner myself: It's NOT happening. Third, what would be the purpose? Some say mind control. But are people acting differently lately? Others say it's to sow disease. But why would anyone want to do this? Who would go along with it? Finally, some say "chemtrails" are a government project to combat global warming. Nice, but then why should such a laudible effort be kept secret? Other web-based "explanations" involve even wackier stuff like electromagnetic rays.

Logic never placates the truly paranoid, and discussions are rarely satisfying. Those who "believe" WANT to believe, and claim soil tests show that dangerous substances have been found beneath the planes. But again, nothing released from 40,000 feet would ever reach the ground except diluted to zero. And, more to the point, the videos of these supposed "chemtrails" shown on the scare web sites are actually a common type of contrail. The believers claim they've only started around 1998 – but I've observed those "spreading out" contrails for over 40 years. They're not new. They're contrails. No mystery, and nothing sinister here at all.