🙄 yeah, spraying burnt/spent jet fuel. Not saying it’s healthy but it’s what happens when water vapour and fine soot particulates from burning jet fuel freeze into ice crystals. In low air humidity, the crystals just dissipate. In higher humidity, they persist, and end up creating visible vapour trails over large areas of sky. Those humidity differences explain why some aircraft are seen producing vapour trails while others do not. Is that too hard to believe?