Chemtrails: Real threat or urban legend?

As an atmospheric scientist, I have sometimes gotten sucked into lengthy, ultimately fruitless arguments with well-meaning folks who deny the evidence for anthropogenic greenhouse warming.

Today, I find myself getting sucked into an equally fruitless argument with well-meaning Facebook acquaintances who are convinced that something called “chemtrails” are being produced by high-flying aircraft, that these are deliberate (either as a form of mind control or as a form of geoengineering to influence the climate), and that these pose a threat to human health.

Here is a sample link representing that viewpoint:

Many believers in chemtrails are so convinced of their position that they believe anyone who doubts in chemtrails’ existence is either utterly lacking in common sense (or “100% crazy”, as someone called me this morning) or else part of the conspiracy.  For that reason, it is probably indeed fruitless for me to itemize the reasons why the evidence FOR chemtrails is extremely weak and why the evidence against them is overwhelming.  But I feel an obligation to do so anyway, mainly for the benefit of laypeople who perhaps haven’t yet made up their minds and are confused by the arguments.

First, I think this Wikipedia article does a pretty good job of summarizing the history of the controversy, so I recommend it for anyone who wants some background.

A key quote near the bottom of the article is from a respected colleague I’ve met and whose work I know well:

“Patrick Minnis, an atmospheric scientist with NASA’s Langley Research Center in Hampton, Virginia, is quoted in USA Today as saying that logic is not exactly a real selling point for most chemtrail proponents: “If you try to pin these people down and refute things, it’s, ‘Well, you’re just part of the conspiracy’,” he said.”

Now, let me be clear: I’m not a head-in-the-sand kind of guy.  There are conspiracy theories (e.g., rigged electronic voting machines) that I believe are not only plausible but likely.  Others, such as the WTC conspiracy theory, I’m still on the fence about for various reasons.  But the chemtrail theory is so untethered from reality as to be quite unbelievable, at least among those who haven’t already made up their minds.  It’s to the latter that I address this note.

First, let’s take the visible phenomenon itself that is claimed to be evidence of “chemtrails.”   Believers point to the fact that some contrails from high-flying aircraft evaporate quickly, while others persist and grow into expansive sheets of cirrus clouds. It’s the latter they claim are chemtrails.

From a meteorologist’s perspective, there is absolutely no mystery about this.  The lifetime and behavior of contrails depend on the environmental conditions in which they form.  Growing contrails result when the humidity at high altitudes is at or above the saturation point with respect to ice.  So when condensation is injected in the atmosphere at those altitudes, the microscopic particles of ice don’t simply evaporate, they persist and even grow, and they are spread by the winds into broader patches of cirrus cloud.  There have been many scientific journal articles that delve into exactly the hows and whys of persistent contrails.

An analogous phenomenon can be observed at the surface at temperatures below -40 degrees, such as in Fairbanks, Alaska, in the dead of winter.   Normally, in a non-urban setting, the arctic air is very dry.  When you breath or drive a car generating exhaust, the resulting plume of fog dissipates rather quickly.  But if the humidity is higher, perhaps partly as the result of emission from lots of other people and/or vehicles in your neighborhood, the fog plume persists, and eventually the whole area is blanketed in persistent, annoying ice fog.  Just ask anyone who has spent a winter in Fairbanks.   Persistent contrails at high altitudes are essentially the same phenomenon.

So are persistent contrails more common today than they used to be?  Probably yes, for two reasons:  (1) More aircraft are flying around making them in the first place,  (2) stratospheric humidity is  higher overall due in part to residual moisture left by previous aircraft and other anthropogenic sources of water vapor and hydrocarbons (see Fairbanks analogy, above).

So if there is a well-understood physical basis for persistent contrails, what is the evidence that compels chemtrail believers to insist that they are really something more sinister?

Well, there’s the assertions of toxic heavy metal contamination and resulting health effects that are prominent in the first article linked to above.  So let’s assume that the measurements of these toxins and documentation of the health effects are valid.  If they are, then yes, we have a problem with toxic pollution of some kind.  But are chemtrails to blame?  Heavy metal pollution is a known problem of coal-fired power plants, among many other industrial sources.  But chemtrail believers are convinced the pollution is coming not from ground-level sources but rather from high-altitude aircraft, and they base this, apparently, on ground-level measurements of pollutants made immediately after a visible episode of persistent contrails.

Now, I don’t know whether similar measurements were also made when there was NO immediately preceding episode of persistent contrails, but I strongly suspect that in areas where ground-level pollution is being detected, careful systematic measurements would reveal that there is NO statistical correlation with episodes of persistent contrails; that is, that pollution is equally likely with or without observed contrails.  And here’s why I believe that:

High-altitude aircraft generating visible contrails are almost invariably flying in the stratosphere at altitudes of 30,000 to 40,000 feet (about 9 to 13 km).   Meteorologists have known for many decades that air in the stratosphere does not readily mix down to lower altitudes, and vice versa.  Stuff put into the stratosphere tends to STAY THERE for many months to a year or longer UNLESS the particles are large enough to fall with a significant speed.

A large snowflake, for example, falls about one meter per second, which still means it would take about THREE HOURS to fall from 10 km to  ground level if it didn’t evaporate first.  But in those three hours, it’s usually also experiencing strong horizontal winds aloft (e.g., the jet stream), meaning that that fast-falling snowflake could still easily wind up 50-100 miles or more DOWNWIND of where it was deposited in the stratosphere.   Snowflakes falling from jetliners over Madison would probably reach the surface over Lake Michigan or beyond.

Now here’s the important point:  Contrail particles are MUCH, MUCH smaller than snowflakes, and fall MUCH, MUCH more slowly.  You can tell this because they remain mainly in flat sheets in the sky, with little hint of visible “fall streaks”.   An aircraft spraying aerosols at 10 km altitude over Madison cannot possibly have a significant local effect on the air quality in Madison or even Wisconsin or Michigan.  In fact, it’s very likely that that substance, whatever it is, would disperse over an extremely large area and become extremely dilute before reaching the surface at all, probably weeks later.  And it would be basically impossible to TARGET any specific region of the surface — 70% of what you sprayed would probably wind up in the ocean.

So let’s review the main tangible evidence chemtrail believers have for their existence:   (1) Persistent contrails, and (2) locally elevated measurements of ground-level pollution.   The first bit has a well-understood meteorological explanation; the second is far more readily explained by ground-level pollution sources — e.g., power plants and other industrial activities.

Now for some other weak links in the chemtrail case:

1) Motive:  Why would someone spray toxins in the air at high altitude? There are various theories about mind-controlling drugs, but if that’s the goal, drinking water would be far more cost-effective.  Or even trucks driving around cities spraying something out the back as they go.  But aircraft flying at 30,000 feet is an absolutely terrible and stupid way to deliver anything to a ground population unless your goal is to blanket the entire earth with the stuff.  For what?  And at what cost?

Another theory is climate control.  And yes, it’s true that there have been proposals to mitigate the effects of greenhouse warming by creating artificial clouds in the stratosphere.  But I can also tell you that (a) most climate scientists agree this would be a very risky thing to do,  (b) it would be EXCEEDINGLY expensive to do deliberately on a scale that would make a difference, and (c) there is no government agency with either the budget or the incentive to try to geoengineer the climate secretly.  In short, I am quite certain that DELIBERATE geoengineering of this kind is not currently occurring.   At the same time, it has long been known that contrails from commercial jets can possibly have INADVERTENT effects on climate as a BYPRODUCT of burning fossil fuels at high altitude – but that has nothing to do with chemtrails.

2) Opportunity:  The vast majority of high-altitude aircraft creating contrails — including persistent contrails — can easily be observed with binoculars to be civilian airliners owned by a variety of private airlines.  Where would the stuff be stored on these aircraft that is supposed to be sprayed?  What would their incentive be to carry this stuff around and spray it, especially given the added cost of transporting every pound of unneeded weight?  How would the countless civilian workers who maintain aircraft, manufacture jet fuel, etc., etc., not stumble across this vast conspiracy to spray toxins at high altitudes by civilian aircraft for unknown purposes?

To summarize, the tangible evidence that chemtrails are more than an urban legend is exceedingly weak – everything I have seen cited in support of the chemtrail theory falls apart in light of far more plausible and mundane explanations.

My conclusion: If you continue to believe in chemtrails after considering the actual available evidence, it’s not because the evidence for them is compelling or even strongly suggestive but rather because you simply prefer to believe in them.


During my recent visit to Berlin, I noticed that “chemtrails” are apparently a big concern there too — at least to individuals armed with cans of spray paint, as much of the graffiti along the river Spree was devoted to the topic (see below).

UPDATE #2  (Aug. 23, 2013):  I just stumbled across this site that debunks chemtrails more systematically than I did above — definitely worth a look.

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