White lines coming from planes aren’t evidence of ‘chemtrails’

A video of white plumes coming from the tip of a plane’s wings has been shared on Facebook with the caption “Chemtrails come from engines right?”

The post has over 700 shares on Facebook and 8,000 likes on Instagram.

It’s not completely clear exactly what the caption means. It may be referring to the fact that so-called ‘chemtrails’ (the conspiracy theory that the white lines emitted from planes are actually chemicals being purposefully sprayed to harm people) are actually usually contrails. 

Contrails appear when water vapour, either already in the air around the engine or emitted from it, condenses around soot particles emitted from the combustion process due to cold temperatures at high altitude. This forms long thin lines of cloud. They can look different, last for varying amounts of time depending on the exact conditions and aren’t large enough to cause weather on the ground (though they can affect the climate).

So the post may be asking why, if contrails result from the output of the engine, do the lines appear to be coming from the tips of the wings instead, and using this as evidence for ‘chemtrails’.

The answer to this is that these lines of cloud that don’t come from the engines are something slightly different to contrails, caused by temperature and pressure changes and water condensing to leave a trail. This is why you sometimes see more than two lines coming from an aircraft with twin-engine jets.

Lines of cloud that emerge from parts of the plane around the wings are caused by wing tip vortices. Strictly speaking, these are not contrails, but produce a similar effect visually although the lines don’t appear to emerge from the engines.

According to NASA, wingtip vortex trails “are often thought to be a type of contrail but are actually produced from a different process”. They form due to a drop in pressure and temperature when a wing generates lift in very specific weather conditions.

The Met Office describes how they happen: “As an aeroplane wing generates lift, it causes a vortex to form at the wingtip and at the tip of the flaps. These wingtip vortices persist in the atmosphere after the aircraft has passed. 

“The reduction in pressure and temperature across each vortex can cause water to condense, producing a thin line of water droplets that looks just like a contrail.”

Even when these vortices don’t produce the line of cloud behind them causing water to condense, photographs taken of planes taking off or landing amongst smoke show the vortex is there.

The Met Office adds that wingtip vortices are usually seen at low altitude after takeoff or before landing when a plane is moving relatively slowly. These trails also tend to evaporate faster than contrails.

A similar phenomenon can also be seen coming from propeller blades.

We have fact checked many posts about the ‘chemtrails’ conspiracy theory over the years, including false claims that a flight map proves they are real and that they are part of a secret geoengineering plot to manipulate the weather.