Vapour Trails.

[Ming the Merciless]

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Missile Defence I think

 

[DaveReading]

Jet powered aircraft do pollute of course, but the visible trails you can see are water vapour. Hot air hitting cold air under certain conditions = condensation.

With the emphasis on "under certain conditions". And not just jet aircraft.

The Americans did a lot of work in WWII trying to understand what combination of conditions lead to vapour trails, because of the implications for their daylight bombing raids.

 

[classic33]

They fitted the U-2 with a rear view mirror so that the pilot could alter his height and prevent contrails from forming.

 

[slowmotion]

They are actually an aerosol of powerful sedatives employed by The Deep State to keep the citizens under control. A bloke in the pub told me that. He had some other interesting information about a cabal of cannibalistic pizza-munching paedophiles too.