Ever notice when watching a jet airplane fly overhead there is a white streak or stream behind it? This has many words for it, the main two words are vapor trail or contrail. Contrails happen when the air flowing over and under the surfaces of the aircraft create wingtip vortices mix with the moist air caused by the exhaust of the engines. The white streams you see are not engine exhaust. In fact what you are seeing before your eyes is formations of clouds which eventually dissipate after a while.
Think of air as water and think of the sky as a body of water such as a lake, river or ocean. So what are wingtip vorticies? As the aircraft moves through the air it pushes the air out of the way, just like a boat moving through the water. When the aircraft pushes this air out of the way the tips of the aircraft wings disturbs the air and causes the air to rotate like a horizontal tornado. This horizontal tornado is called wingtip vortices and is also termed wake turbulence. Note the word wake in wake turbulence, just like a boat has a wake behind it when moving through the water. Wake turbulence can be a serious hazard to small aircraft landing or taking off after a larger, heavier aircraft.
Keeping this in mind, lets go into a little more detail about contrails. As the aircraft forges its way through the atmosphere the exhaust from the engines mostens the air. The air temperature at these altititudes is below freezing. Moist cold air (water crystals) creates wind creates these contrails which are really just small clouds. So the next time you see these, think, I learned about this on Bright Hub! If you have more questions about aviation, aeronautics or airplanes you can post them in the Discussion areas found in each channel, or Private Message the author.