For as long as I can recall on my time on the internet, there has been much speculation as to what exactly those vapor streaks behind jets were – rather, what we see on a sunny day floating around is some mysterious gas, maybe. Or perhaps, its a chemical! ...right? Well, lets look into this, and dig into the science behind these vapor trails.
In order to what's going on when you see these mysterious wisps in the sky, lets take a common example down at our level. Have you ever breathed onto a window in the cold? Take note of how when your breath comes into contact, the area around where you breathed condenses and moisture forms atop the surface. The environment must be very specific in order for this to occur. Using this breath example, a 98 degree mouth breathing upon a cold window on a cold day would, expectedly, form condensation, and this particular scenario is one of the few scenarios in which this would occur: that source of moisture (your breath) must be magnitudes warmer than the surrounding environment, and the aforementioned environment must have sufficient moisture in order to condense. Inversely, we all know what happens when you take a cold can of soda outside on a hot and humid summer day. This principle is where these condensation trails - or contrails - come in.
When airplanes are at cruising altitude, (around 30,000 feet, on average) the atmosphere they fly throughout is often very, very, cold. Temperatures at this height can range from 20F to as cold as -50F, compared to around 60F on the ground back down here. When the supercooled environment of that atmosphere interacts with the stupendously hot engines, often churning and running above 400F, the air in the immediate wake of that toasty engine is briefly heated and moisture condenses, just like our past examples – a Contrail is born. Literally speaking, contrails are just clouds!