One of my favorite conspiracy theories to debate is "chemtrails." The factual explanations behind the puffy white lines are so fabulously simple, you've got to marvel at those who harbor this preposterous notion. Entertain no fear, intelligent reader, as this conspiracy can only be held by the least scientific among us. To argue with chembelievers is to feel both frustration and bewilderment manifest.
You'll hear the battle cry of the Chemtrailers: "Wake up! Look up!" soliciting you to abandon your ability to research for the blind acceptance of anecdotal opinion. We live in a world where information is so readily accessible for anyone who chooses to pursue it. The challenge comes in vetting sources, and this seems to be the trap in to which Chemtrailers fall. They want so badly to be right about being sprayed, they will use any source available that serves their confirmation bias.
Contrails, as they're known by the scientifically literate among us, are quite simply explained. In fact, NASA does quite a good job of expounding it:
"Contrails are clouds formed when water vapor condenses and freezes around small particles (aerosols) that exist in aircraft exhaust. Some of that water vapor comes from the air around the plane; and, some is added by the exhaust of the aircraft. The exhaust of an aircraft contains both gas (vapor) and solid particles. Both of these are important in the formation of contrails. Some elements of the exhaust gasses are not involved in contrail formation but do constitute air pollution. Emissions include carbon dioxide, water vapor, nitrogen oxides (NOx), carbon monoxide, hydrocarbons such as methane, sulfates (SOx), and soot and metal particles." 
Now the fun part...
"THERE'S CHEMIKILLS IN MY AIR."
The quintessential Chemtrailer will claim that there are a host of chemicals being sprayed on us. In my experience, the most common particulates mentioned are aluminum, strontium, and barium. If these were being littered upon us in such volume as to cause detriment to our health, they would be easily detectable in soil and air samples yet, not surprisingly, no proof has been offered from any laboratory to date. Ask the conspiracy theorists to provide one; they can't and they won't.
"CONTRAILS DISSIPATE, BUT CHEMTRAILS DON'T."
Some of the conspiracy theorists don't want to seem as crazy and so they'll justify their position by saying that chemtrails stay in the sky for hours while contrails dissipate quickly.
This is simply untrue according to NASA:
"Both the temperature and humidity of the atmosphere help to determine whether a contrail can form. As the relative humidity in the atmosphere increases, the atmosphere is able to supply more moisture into the jet aircraft exhaust plume, and a contrail is more likely to form. The temperature of the atmosphere does not have to be as cold to form contrails at 60% relative humidity compared to 0% relative humidity. If the relative humidity of the atmosphere were 60 percent, then a contrail can form if the atmospheric temperature is less than the temperature indicated by the 60% line (temperature profile is to the left of the 60% line)." 
That's all well and good, Chemtrailers, but if our air is being poisoned, wouldn't the people who are spraying us also have to breathe the same air? Don't their family and friends also breathe this air? Why would these people poison themselves and their friends and family? These are questions I have never had answered by any conspiracy theorist.
"THEY'RE TRYING TO CONTROL THE WEATHER AND OUR MINDS WITH HAARP."
The High Frequency Active Auroral Research Program (HAARP) will invariably be mentioned at some point in any given debate with a chemtrailer. They drop it like a syllogistic bomb as if the acronym itself is all of the proof that they need to evidence that we'll soon be mindless zombies wandering the Earth. Like so many other things, conspiracy theorists cling to things that they don't understand as evidence of whatever hypothesis they are ranting about.
Arguably the most respected peer-reviewed journal in the world, Nature had this to say about HAARP's operation:
"It's a Strangelovian scenario that only the Pentagon could dream up: North Korea, in the throes of a military coup, launches a nuclear weapon that explodes 120 kilometers above the Earth. The blast fills the atmosphere with 'killer' electrons that would within days knock out the electronics of all satellites in low-Earth orbit. It would cause hundreds of billions of dollars of damage, and affect military, civilian and commercial space assets.
If this doomsday scenario sounds outlandish, then the possible response may sound even more improbable: injecting radio waves into the atmosphere to force these energetic electrons out of orbit. Yet this is exactly what the US Department of Defense is looking at in a major ionospheric research facility in Alaska."
"HAARP's purpose is to study the ionosphere (the section of the atmosphere beginning about 50 miles up in which ultraviolet radiation temporarily strips atoms of their electrons), the magnetosphere (the region in space above the ionosphere where the Earth's magnetic field affects the behavior of charged particles) and the Van Allen radiation belts (bands of highly charged particles contained in the magnetosphere beginning some 400 miles up). Scientists are interested in the ionosphere because of its ability to affect radio signals; the Van Allen belt, because the radiation there damages satellites, and a better understanding of it could lead to ways to make satellites last longer."
I do not write this as the be-all end-all destruction of Chemtrailers because this article does not even skim a fraction of the scum from the surface of the conspiracy pond, but it covers a few of the preeminent whines from those who fear cirrus aviaticus, the contrail.
Be skeptical. Audit sources. Suffer no fools.
Editing: Elyse Carabello
Image courtesy of John O'Neill. This image was released under the public domain.