Air & Space Magazine tells the story of condensation trails, or contrails, the white streams of ice crystals generated by airplanes flying at high altitudes. From the article:
They are created by airplanes flying at high altitudes, where the air is below –38 degrees Fahrenheit. Exhaust from airplane engines contains water vapor as well as other gases and particles of soot and metal. When the exhaust is expelled into and mixes with the cold air, the water vapor condenses into droplets, which instantly freeze into tiny ice crystals…
It's not only jets that make contrails; piston aircraft do too. So do rockets. So, apparently, do birds. "I have heard of wild geese leaving vapor trails high over the Canadian Rockies," Guy Murchie wrote in his book Song of the Sky. A goose exhaling warm, moist air into –38-degree air could produce a contrail, Minnis allows, although "it would certainly be a small one…"
There are those who consider contrails to be downright sinister: noxious chemicals sprayed from aircraft to sicken populations and alter weather patterns, according to conspiracy theorists. The claims seem to rest on the notion that thin, short-lived contrails may consist of ice crystals, but the thicker, long-lived ones are not. In reality, the expanded lines are merely contrails that have evolved.
Previously on BB:
• Airplane exhaust scars Link
UPDATE: Kate Green writes:
Turns out that contrails act like artificial cirrus clouds, and their
presence actually affects regional temperatures. Like cirrus clouds,
they cool the earth during the day by blocking out sunlight and warm the
earth at night by trapping heat. A Penn State researcher determined
contrails make a difference of about 2 degrees after the attacks on
September 11, 2001 when planes were grounded; for the first time he was
able to directly compare temperatures logged in the presence of
contrails against temperatures collected in contrail-free skies. I wrote an article about it here.