HomeIndustryAerospaceWhy Do Some Planes Leave White Trails In The Sky?

Why Do Some Planes Leave White Trails In The Sky?

You may have noticed, on a sunny day, some white lines on the bright blue sky and you realized (or not) that planes are the source. But have you ever wondered how they are formed and why?

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Adrian Pingstone / Wikimedia

All jet engine airplanes produce white smoke, does not matter how many engines they have. But it is smoke or it is something else? We’re going to take a closer look at this phenomenon.

What are contrails?

The white tracks planes leave behind are artificial clouds, and they’re called contrails, short from condensation trails. You can easily track an airplane’s path because the white trail it leaves behind, in the right humid atmosphere, can stay in the atmosphere for hours.

How contrails are formed

They do not always appear because they require certain atmospheric conditions. Those tracks are forming similar to how, on a cold day, the breath you exhale can condense into vapors. There are three types of contrails: short-lived, persistent non-spreading, and persistent spreading.

Jet planes fly at an average altitude of about 30,000 feet, where the air is colder and not very dense. The air produced by a jet engine is warmer and more humid than the air where those airplanes fly (sometimes even more than -40°C/-40°F), the mixing of these two types of air triggers various changes, such as condensation. In this process, the water vapor gases turn into small water droplets or even tiny ice particles resulting in the formation of the contrails, the cloud0like tracks you see in the sky.

The effect on climate

The aviation sector is responsible for around 2.5% of global CO2 emissions and the effect on the Earth’s climate of contrails could be more significant than we thought. Some scientists calculated that over 55% of the entire climate impact of aviation is due to contrails. As contrails sometimes form clouds in the atmosphere, they can affect the radiation balance of our planet, trapping and absorbing outgoing heat which otherwise escapes into space. 

Chemtrail conspiracy theory

Airplanes leave behind a trail of white smoke due to the mixing of hot and humid air with cool and dry air, but some people mistakenly believe that the white trail is a chemtrail.

Conspiracy theories about chemtrails started to circulate after a 1996 report about weather modification was published by the United States Air Force (USAF). They were blamed for “spraying the U.S. population with obscure substances” from planes “generating unusual contrail patterns.” In short therm, the chemtrail conspiracy theory spread, and federal officials were overflowed with furious calls and letters.

Skywriting or Skytyping

Skywriting of 'Sorry' above the Sydney Harbour Bridge

Ever sat at an outdoor event and watched an airplane writing in the sky? Is that contrail? Not actually.

Skywriters use small airplanes equipped with special smoke machines. The typical smoke generator consists of a pressurized container full of oil. This oil is injected into the hot exhaust manifold, vaporizing it into a huge volume of dense white smoke. Pilots must learn how to fly special patterns from advertisements to marriage proposals.

Skytyping is a technique whereby the smoke is emitted in a series of bursts by a computer that generates electronic signals to control the smoke output.

Fun facts about contrails and skywriting

  • A scientist named H. Appleman published in 1953 a chart that was successfully used to determine when an airplane would produce a contrail.
  • The first skywriting for advertising was done in 1922.
  • A letter can be as high as 1 kilometer and take 60-90 seconds to create.
  • A message can stretch up to 25 kilometers (15 miles).
  • The best conditions for skywriting are a clear sky, no wind, and cooler temperatures. Then the letters may be seen for 50 kilometers (30 miles) in any direction and can last 20 minutes.
  • Writing occurs usually at altitudes from (2,000-5,000 meters) 7,000-17,000 ft.
  • The paraffin oil vaporizes at 1500° is tottaly safe for the environment.
  • One company in New York writes more than 50 marriage proposals a year in the sky.
  • In 1960, skywriting and skytyping were banned by the government because of concerns over the potential spread of political propaganda.

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