Herbert Appleman’s nomogram, 1953. Temperature and pressure measurements falling in the region to the left of the 0 percent humidity line could be assumed to constitute contrail-forming conditions, according to the Appleman model, while the region to the right of the 100 percent line could be assumed to be contrail-free. For the intervening zone, contrails would be assumed to form behind a passing jet at a given pressure (altitude) if the temperature fell to the left of the relevant line indicating the observed ambient relative humidity.
This 1950s nexus of military-industrial paranoia, anthropogenic cloud monitoring, and civilian sky gazers has proven strangely durable—if also unsettlingly plastic. For about a decade now, a vast conspiracy-theory subculture out there in the interwebs has promoted, with stupefying energy and commitment, the notion that some, many, or even all visible contrails are in fact properly understood as chemtrails: miasmatic evidence of a sweeping government plot to deploy aerosol spraying across the country (and perhaps the world). Many argue that this has something to do with climate change geo-engineering, but others allege large-scale human experimentation, genetic manipulation of populations, and possible military objectives. It is a passionate community, if somewhat shadowy, and at times genuinely enraged. For all that, its partisans have had some success in forcing their issue to the edges of respectable investigative reporting and non-fringe documentary filmmaking.
I could be wrong, of course, but I spent some time with the available material, and it seems that there is nothing to it at all. Contrails are contrails. They have been around for a long time. They are not new, and the things that are streaming out of the airplanes outside my window are contrails. There is a lot of good evidence for all this. While from time to time people do spray stuff from airplanes, and while this has surely been, on a number of notable occasions, a very bad idea, commercial airlines are not being deployed in a secret, large-scale plot to defeat our enemies, willfully transform the earth and its inhabitants, or poison us by nefarious means. While contrails themselves, it turns out, may indeed be slightly altering the weather (because they actually slightly add to the earth’s total cloud cover), this has absolutely nothing to so with so-called chemtrails, which, in a basic sense, do not exist.