The Great Contrail Controversy

Back when I was studying physics, there was a big controversy about contrails - those white lines that aeroplanes leave behind them in the skies. It might have been sorted out now; I'm not sure, but I do think I know the answer.

There were two main schools of thought when I was studying physics. One school of thought said that they were caused by the engines - the air coming out of aircraft engines has more water vapour than the air going into them (because burning fuel creates carbon dioxide and water) and therefore the contrails were that water vapour condensing behind the aircraft. The problem with this is that some aeroplanes have three engines - one on each wing and one on the back, and they don't seem to leave three-lined contrails.

The other school of thought was that because air goes faster over the top of the wing than the bottom of the wing, the aircraft moving through the air creates vortex strings behind it, and water vapour in the air condenses onto them. The problem with this is that it's not immediately obvious that it works as an idea...

I was flying back from Rome a while ago, and I noticed contrails forming behind the aeroplane, and I noticed where they were coming from. On the back surface of the wing, there are flaps. And when the flaps are lowered, this creates a jagged back surface to the wing, as if some bits were missing. The contrails were forming from the edge of those jagged bits - they're created by turbulence from air coming off the back of the wing where the wing is uneven because of flaps.

That's what caused these anyway...