What causes vapor trails from jets? — Larry, Durham, N.C.
Marshall Brain Answers:
First, a gallon of jet fuel is made up of molecules that are carbon chains. A typical jet fuel molecule is C10H22. When the fuel burns in the engine, the carbon atoms in the chain combine with oxygen atoms in the air to create carbon dioxide (CO2). The hydrogen atoms combine with oxygen to form water (H2O). In rough terms a gallon of jet fuel produces approximately a gallon of water when it burns, but that water is hot steam as it comes out of the engine.
Second, a big jet burns lots of jet fuel. A 747 is burning about a gallon of fuel every second. So there is a gallon of water exiting the engines every second in the form of steam.
Third, at 30,000 feet the air temperature might be -40 degrees F (-40 degrees C). So the steam comes out of the engine, condenses into water and freezes into ice almost instantly. The vapor trail you see is that condensation, known as a condensation trail.