Let me start bluntly: it’s NOT smoke. These trails that are sometimes visible behind planes consist simply of water vapour and are known as Condensation Trails (Contrails for short). Contrary to popular belief, these are not deliberately released- whether it is a form of advertisement for the airline, airplane tracking for officials on the ground or even to cool the engines. In a manner of speaking, contrails show up whenever they want, but it isn’t really that simple!
Because contrails consist of billions of ultra-fine water droplets in suspension in air, the main factors affecting their formation are Temperature and Humidity; to be specific, low temperature (<-35⁰C) and high humidity
(>80%), making it easier for the vapours to condense and reflect sunlight, making them visible.
Where do these vapours come from?
As mentioned above, visible trails tend to form only when the air is more than 80% water vapour, however, water is a major by-product of the combustion of fossil fuels in the engines of an Aeroplane, along with Carbon Dioxide. Due to this, contrails have similar properties to natural clouds in terms of persistence and dissipation.
There are 3 types of contrails:
This is the most common form of contrail, and typically the longest lasting, sometimes stretching over many kilometres for nearly 6 hours! These form solely at high altitudes, due to the suitable conditions.
What triggers their formation?
Their formation is triggered partly due to the sudden increase in water vapour due to the engine but mainly because the particulates in exhaust fumes stimulate the condensation of the water vapours, to form clouds trailing behind the aircraft engines. Due to the cohesive properties of water, the trails behind each engine tend to join, eventually forming a single large trail.
Unlike exhaust contrails, aerodynamic contrails form behind wingtips and any deployed control surfaces of the plane. They form more commonly at low altitude and speed due to the high ambient humidity.
What triggers their formation?
These are triggered when there is a sudden drop of pressure due to changes in altitude of the craft. The column-like shape of these is due to the vortices formed by varying speeds of air around the wing. Notably, these are also known to form within the intake of Turbo-Fan engines (Common on most commercial Jets) during takeoff. This is due to the blades reaching Transonic Angular Velocities, significantly dropping air pressure.
As you have probably guessed by now, these are a combination of exhaust and aerodynamic contrails, and form only when conditions are suitable for both types to form within a few metres of one another, making them quite rare.
The difference between a Hybrid and Exhaust contrail is visible with time, as the significant difference is the “Wake Vortex Core”, where pressure is considerably low. This core is only visible when the surrounding exhaust contrail has dissipated. Due to this low pressure core, the ice crystals in this zone combine to form larger and
larger ice crystals, giving the impression of a contrail that disappears into itself.
As contrails consists of water vapour, they sometimes refract water, creating beautiful rainbow-patterned clouds behind an aircraft.
Due to the challenges of using natural contrails, skywriters use artificial smoke to form long-lasting text in the
sky. This typically consists of the injection of a cold, low viscosity oil onto the ht exhaust manifold. This oil is also sometimes dyed for airshows.
Chemtrail Conspiracy Theory
Don’t you just love these? First the Moon Landing Conspiracy and now Chemtrails, it’s as if the world never runs out of speculation! This theory insinuates that certain powerful people are intentionally spraying chemical and biological agents into the sky, for an arsenal of reasons! However, this has been repeatedly (and very harshly) debunked by everyone from scientists, to governments, to pilots!