About a month ago, I gave a presentation in Portland in which I described my prediction for the upcoming winter. Afterwards a lady asked a question I’ve heard plenty of times — but not for awhile. She asked, “How do chemtrails affect climate?”

Mind you, not contrails but chemtrails. Contrails (short for “condensation trails”) are visible trails of condensed water created by high-flying aircraft. Water vapor is a major component of aircraft exhaust, and when the vapor comes in contact with the very cold air at high altitudes, it condenses, forming clouds. The clouds may be made up of tiny water droplets or very small ice crystals, depending on the temperature. Contrails most often occur at heights above 25,000 feet and at temperatures of less than minus 40 degrees.

But about 12 years ago a theory was developed that suggested that some contrails were being intentionally created or enhanced. The “chemtrail” theory suggests that some contrails are caused by chemical or biological agents deliberately sprayed at high altitudes. The purpose of such spraying remains a mystery, according to this conspiracy theory.

The chemtrail theory spread quickly after its genesis, thanks to the Internet and radio talk shows. Thousands of complaints and questions were sent to federal agencies, which have consistently denied any involvement.

Chemtrail buffs differentiate chemtrails from ordinary contrails by reporting that they sometimes persist in the sky for hours, often with crisscrossing, gridlike patterns or parallel stripes that merge together and form large clouds. Other features that distinguish a chemtrail from a contrail are the presence of colored prisms in the trails; large numbers in a single area; or lingering tracks left by unmarked or military airplanes flying in atypical altitudes or locations.

One of the publications that inspired the chemtrail theory was a 1996 report by the Air University called “Weather as a Force Multiplier: Owning the Weather in 2025” in which modification of weather was viewed as a possible offensive and defensive weapon in time of war. The report stated that by 2025 it might be possible to cause flooding, cause drought, modify or enhance storms, and increase or decrease clouds or fog, to name just a few potential applications. Each of the above could have profound military effects. The Air Force denies it is conducting any experiments along these lines.

According to some accounts, chemtrails result from such chemicals as barium and aluminum salts, polymer fibers, thorium or silicon carbide. Others allege that the skies are being seeded with electrically conductive materials as part of a large upper-atmosphere research program called the High Frequency Active Auroral Research Program. The latter inspired a conspiracy book called “Angels Don’t Play This HAARP.”

Why would the government resort to such measures? Those who believe in the conspiracy theory list military weapons testing, chemical population control and measures to mitigate global warming as possible reasons.

In 2001, Congressman Dennis Kucinich introduced a bill that would have prohibited space-based weapons. Chemtrails were listed as one of a number of possible weapons that would be banned. The bill received an unfavorable evaluation from the Department of Defense and never made it out of committee. Subsequently, Kucinich tried three times, without success, to pass a Space Preservation Act, but chemtrails were not mentioned.

A couple of references I’d suggest if you want to learn more:

Chemtrail Central: www.chemtrailcentral.com

Chemtrail Controversy: www.lightwatcher.com/chemtrails/Chemtrail_Controversy.html

George Taylor retired as state climatologist earlier this year and now operates Applied Climate Services of Corvallis. He can be reached at taylorgh@comcast.net.