Trail of Mist Behind Airplane Wingtip

Q: Sometimes when landing, I see a long thin trail of mist coming from the wingtip. What is this? My friend says it’s fuel being dumped to lighten the plane.

Maybe that friend is the same person who told another reader that the contents of airplane toilets are jettisoned into the sky after flushing. What you’re seeing is moisture, condensed as the airflow whips around the wingtip. Wingtips spin off an invisible vortex of air — something like a miniature sideways tornado — and this moisture stream is, in part, a visible manifestation.

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This Q&A is part of a collection that originally appeared on Salon.com. Patrick Smith, 38, is an erstwhile airline pilot, retired punk rocker and air travel columnist. His book, Ask the Pilot (Riverhead) was voted “Best Travel Book of 2004” by Amazon.com. Patrick has traveled to more than 55 countries and always asks for a window seat. He lives near Boston.