I'd heard of the "chemtrails" idea before, but it wasn't until I started coming to Newsvine in 2011 that I realized that it was being taken seriously among a surprisingly large number of people. I had several discussions with people here, looking for the empirical evidence that supports this idea, and finding none. Recently, I decided to take it to a web forum devoted to "chemtrails". I registered at Global Sky Watch, and posted the message below on their "science" forum. Shortly thereafter, my thread was deleted, and I was permanently banned. The banning message read: "The existence of chemtrails has been well established and is considered trolling and is not allowed on this forum" (sic). I'll invite the administrator there (by email) to comment on this article.
I'm glad that this forum has a science section. I'd like to discuss the empirical evidence behind "chemtrails". Over the past year I've run into this idea on other discussion websites, and I've been asking for empirical evidence (i.e. from observation or experiment) for the existence of "chemtrails". Generally, what I get in response are links to videos and websites which start from the premise that "chemtrails" are real, and then attribute other phenomena and experiences to this alleged spraying. Let me make it clear that I won't look at a video in response to this post, unless you can explain in your own words what empirical evidence is presented in the video, and where in the video (minutes, seconds) it starts to be presented. I've sat through a lot of these videos, waiting in vain for the promised empirical evidence to appear.
Generally, there are two main types of empirical observations that I've seen referred to as evidence for the existence of "chemtrails".
1) Pictures and videos of persistent contrails.
A central belief behind the "chemtrails" idea is that normal contrails cannot persist for more than a few minutes, and thus if clouds behind jets persist for hours and form artificial clouds, then they are actually the result of intentional spraying of chemicals, particulates, nanobots, etc. Thus, those looking for evidence are told to "look at the sky", days where there are a lot of such phenomena are called "spray days", etc.
However, this belief is completely false. Normal contrails can indeed persist for hours and gradually spread into artificial clouds, when the atmospheric temperature and moisture conditions are right (low temperature, high relative humidity) for ice crystals to form. The mechanism for this was first described by Appleman (1953), and scientific understanding of it has since been refined (e.g. Hansen and Hansen (1995) and Schraeder (1997)).
Persistent contrails have been observed and documented for about as long as high-altitude air traffic has existed. They were a significant issue for WWII pilots, who had to fly at higher altitudes than was normal for passenger planes at that time, to avoid enemy fire. The persistent contrails gave an easy way for them to be spotted in the sky, and caused visibility issues for the pilots. See here for an album of WWII persistent contrail images. [Edited to add: see also the interesting article, "Wakes of war: contrails and the rise of air power, 1918-1945" for an in-depth exploration of the importance of persistent contrails during that period.]
The effect of these artificial clouds on weather and climate has also been a subject of scientific study for decades, and it continues to be so. For example, consider this 1970 paper (with contrail pictures from 1969) in Journal of the Atmospheric Sciences: "The spreading out of jet contrails into extensive cirrus sheets is a familiar sight. Often, when persistent conditions exist from 25,000 to 40,000 ft, several long contrails increase in number and gradually merge into an almost solid interlaced sheet."
Some people find it remarkable (and perhaps ominous) that these persistent contrails can be so abundant in the sky, forming parallel and grid-like patterns. But this is not surprising, considering just how much jet airplane traffic is in the air at any given time, and the fact that they tend to fly along certain routes that naturally lead to those patterns (especially nearby airport hubs and major air traffic lanes). See the NATCA.org flight explorer to view how many planes are in the sky right now (with a 5-minute delay). Here's a really fascinating video made by an artist from 24 hours of flight traffic data (which demonstrates why these grid and parallel patterns would be expected):
24 Hours of Flight Data
For more information please see:
2) Soil and Water Tests for Aluminum, Barium, and Strontium
Although ideas of what the "chemtrails" would contain appear to vary (including such things as nanotechnology and pathogens for population control), the most common idea is that they are spraying aluminum, barium, and strontium compounds as part of a secret program (for geoengineering, weather control, or other purposes). Believers often post the results of water tests from rainfall, streams, and ponds, as well as soil samples, showing different levels of these elements, as supposed evidence of "chemtrail" deposits.
The problem with this claim is that these elements are a normal part of soil and airborne dust. Aluminum is the most abundant metal in the earth's crust, and commonly makes up between 1% and 30% of soil mass (about 7.1% on average)(source). Barium is found naturally in soil at about 300 to 1,000 ppm (source). Many sources say that strontium is present at low levels nearly everywhere in the environment - according to wiki it makes up 0.034% of all igneous rock.
So, in order to show that the levels of these elements in samples are remarkable, they need to be compared to some baseline of what is normal. Many "chemtrails" activists seem to use EPA drinking water standards, but it does not make sense to expect dirty pond or river water to meet strict drinking water standards. A 1967 Canadian preliminary study found that aluminum in rainwater ranged from 520 ug/L to 1,120 ug/L across 13 samples (source). Barium is normally found in surface waters at levels between 2 and 340 ug/L (source). River water commonly contains about 50 ppb strontium (source). Naturally, the amount that gets into water is going to vary depending on the local soil and rock composition, the level of dust in the air, weather patterns, etc.
Even if it could be established that the level of these elements in samples is above the range that could be expected from natural sources, it still would need to be demonstrated that "chemtrails" are the source. Ideal evidence would include samples taken from the trails themselves, but if that's not possible, one could at least monitor the levels of these elements deposited over time, and see if they actually increase following the alleged "spray days" (which are more conventionally understood as days when persistent contrail conditions exist), and decrease at other times. I have not seen any "chemtrails" activist attempt to perform such systematic empirical data collection.
In summary, pictures and videos showing phenomena that appear identical to conventional persistent contrails are not evidence for the existence of "chemtrails". Soil and water samples showing elements that are common in soil and dust are not empirical evidence for the existence of "chemtrails", particularly without any indication that these measurements are unusually high. So, can anyone here offer anything else?