Three things are needed for them to form: water vapour, cool air and particles on which the water vapour can condense. Water vapour is produced by planes as the hydrogen in its fuel reacts with oxygen in the air. In cold conditions (typically below around -40C (-40F)) it can condense, typically on the soot particles also emitted from aircraft engines, to a fog of droplets, which then freeze to form ice particles. The process broadly resembles frozen breath on a cold winter's day, says Schumann.
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