The earliest known account of a contrail (short for condensation trails) was in 1915 – only 12 years after the Wright Brothers first achieved powered flight. They formed because, like aircraft of today, they used hydrocarbon fuel to propel themselves through the sky. The product of this combustion is mostly carbon dioxide and water vapour which can raise the relative humidity in the local area past the saturation point. Soot particles from the combustion process and other condensation nuclei then start to accumulate the water vapour and grow to little droplets which in the cold air immediately freeze. Depending on the conditions, these tiny ice crystals can attract more water from the air resulting in thicker contrails. These contrails usually form at high altitudes but they can also form closer to the ground when the air is very cold and has a high humidity. This is why it took over a decade for this first to be reported, as the first aircraft were not reaching the altitude needed for their formation.
Depending on the air conditions, the contrail may only exist for fractions of a second or it might last for several hours. The difference is due to the dryness of the air, as if the air is relatively dry the ice crystals will sublimate quickly back into vapour, whereas if the air already has a lot of water vapour in it ,it will take much longer.
As you can imagine, contrails became a massive issue during the second World War when aircraft were having to fly at high altitudes in order to avoid anti-aircraft fire. They could easily be spotted and even followed by enemy aircraft.
This should really put a spanner in the works for people who believe in chemtrails, which is the conspiracy theory that some of the lines left in the sky are actually chemical or biological agents deliberately sprayed on the population for nefarious reasons. One of the key cornerstones of this conspiracy movement is that there is an easy to spot difference between contrails and chemtrials, which is the time they linger in the sky. According to the people I met in London at the March Against Chemtrails and Geoengineering protest, contrails should only last a few seconds before disappearing whereas chemtrails last for hours.
So how could they explain away the fact that white lines came from behind military aircraft during the Second World War that lingered for hours… that, strategically, would be madness to continue doing during a conflict. To find out, I decided to ask one of the protesters down in London.
To my surprise, when confronted and the war was mentioned, the protester said that yes, indeed spraying of chemicals was happening during the Second World War. I have to admit I was a little flabbergasted and after the interview spent the next 10-15 minutes trying to get my head around how anyone could believe that at the height of the Second World War, both sides would give themselves such a tactical disadvantage and risk losing the war by continuing to spray chemicals on the population using propeller planes.
Even to this day, it is something that I have never been able to figure out, and shows a complete disconnect from reality that I have never seen before.