Expert: Paul Soderman - 4/10/2007

Question
Always wondered why the Jet planes leave a long stream of smoke whiich is visible from the ground as two lines. Why this is happening, are they wantedly doing this or any other special means for doing it. Thanks in advance.

Answer
Good question Mohan.  What you are seeing is the wake of the airplane, which is a natural process much like the wake from a boat.  The wake is not really smoke, but condensed water vapor, water vapor being a product of combustion from a jet engine.  This happens only at high altitude where it is very cold.  Aircraft wakes roll up into wing tip vortices, which are swirls of air much like tornados.  Those create the two lines that you see.  Vortices are created by the aircraft lift and speed. I had a colleague at NASA who spent a career trying to find a way to minimize those vortices since they are a hazard to following aircraft, but elimination is not yet practical.  It is kind of like trying to eliminate the wake behind a boat.  Not easy.  But we now know a lot about how they are formed, where the move, and how the dissipate.  For those of us on the ground it is not a worry.
Paul