Contrails at sunrise over South London (Tim Ireland/PA)

Condensation trails created by aircraft in the sky are harmless

By Flora Bowen, PA
9:40 - April 26, 2023

Online claims suggest planes are spraying people with harmful chemicals, which look like condensation trails (“contrails”) in the sky. It is claimed “chemtrails” can be distinguished from contrails because they last longer in the sky.

Evaluation: False

The vapour trails seen behind planes as white lines in the sky are “contrails”, short for “condensation trails”. They are mainly made of water in the form of ice particles.

The facts

What are contrails and how are they formed?

The Department for Transport says contrails are caused by condensation, due to changes in air pressure.

Condensation is a process by which water vapour becomes liquid. As warmer air from an aircraft exhaust mixes with cooler air around the plane, the air cools and is less able to hold moisture. The air condenses to form vapour, which appears as a line-shaped cloud in the sky.

Duration of contrails

The length of time a contrail remains in the sky depends on the atmospheric conditions surrounding the aircraft, according to the Federal Aviation Authority. In very dry air, the vapour in the form of ice crystals turns back into water vapour and disappears in a few minutes.

In humid air the ice crystals remain and may create a wider contrail that may look like cirrus clouds.

Why some planes do not have contrails

Newer and more efficient engines with cooler exhaust gases can cause the formation of contrails at lower altitudes than those of less efficient engines. This means some aircraft may not leave contrails even at the same altitude as those which do.

The Department for Transport says this scientific phenomenon explains images of an Airbus A340 leaving contrails, and a Boeing 707 (maiden flight in 1957) not leaving contrails, while both flew at 33,000 feet.

Safety

There is no danger to humans, according to the Federal Aviation Authority. The contrails predominantly consist of ice particles with a small portion from the engine exhaust. The engine exhaust contains products of combustion of aviation fuel that are like the exhaust composition of cars.

As the combustion takes place between 7,000 to 8,000 metres above the earth’s surface, the contrails do not reach the Earth’s surface.

While jet engines emit “carbon dioxide, small amounts of un-burnt hydrocarbons, oxides of nitrogen, soot particles (sometimes termed non-volatile particulate matter, or nvPM) and carbon monoxide” as well as water vapour (“the primary exhaust emission”), the Department for Transport said: “There is no evidence that contrails cause health problems.

“At the altitude that contrails are produced, the oxides of nitrogen and particulate matter which can cause respiratory problems become dispersed.”

What are “chemtrails”?

“Chemtrails” refer to a debunked conspiracy theory which claims the white lines behind aircraft are evidence of the population being sprayed with chemicals, with various suggested reasons ranging from weather modification or geo-engineering to spreading poison or vaccines.

Links

A claim on Twitter (archived tweet and video)

Second claim on Twitter (archived)

Contrails Q&A – Gov.UK (archived)

Contrails 101 (archived)

1EPA Aircraft Contrails Factsheet (archived)

Influence of propulsion efficiency on contrail formation (Fig 7) (archived)

BBC News fact check on chemtrails (archived)

USA Today fact check on chemtrails (archived)

Full Fact fact check on chemtrails (archived)

AFP fact check on chemtrails (archived)

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