Below 20k feet. Clouds dont sudden 20 degree turns either. The chem blends, different recipes are mixed into the jet-a, just like Prist,(anti-fungal & to keep fuel from freezing @ altitude)is blended in. This is not vapor.....nothing makes contrails below 30K......its not cold enuf to freeze hot exhaust. Its, ok, the smart phone will prevail in denial.
Winds Aloft are different at various altitudes. They can be from calm, up to over 150 mph. You can have winds coming from 360* at 100 kts at 20K ft, and be from 260* at 50 kts at 15K ft. This explains the dispersal, and the occasional change in direction. Remember, aircraft DO climb, descend, and manuever laterally at altitude. The fact is that contrails can exist at ANY altitude, given the proper combination of temperature, humidity, and air density. Ever seen a jet take off from a COLD location, like Anchorage or Fairbanks? I have. At minus -20*F, contrails do exist as they takeoff. The exhaust does not "freeze", it is the temperature differential of the hot exhaust gases as they hit the cold air that causes the condensation.
The book definition of contrails, as shown in wiki:
Contrails (
/ˈkɒntreɪlz/; short for "condensation trails") or
vapor trails are line-shaped
clouds sometimes produced by
aircraft engine exhaust, typically at aircraft cruise altitudes several miles above the Earth’s surface. Contrails are composed primarily of water, in the form of ice crystals. The combination of water vapor in aircraft engine exhaust and the low ambient temperatures that often exists at these high altitudes allows the formation of the trails. Impurities in the jet exhaust from the fuel, including
sulfur compounds (0.05% by weight in jet fuel) provide some of the particles that can serve as sites for water droplet growth in the exhaust and, if water droplets form, they might freeze to form ice particles that compose a contrail.[SUP]
[1][/SUP] Their formation can also be triggered by changes in air pressure in
wingtip vortices or in the air over the entire wing surface.[SUP]
[2][/SUP]
Depending on the temperature and humidity at the altitude the contrails form, they may be visible for only a few seconds or minutes, or may persist for hours and spread to be several miles wide, eventually resembling natural
cirrus or
altocumulus clouds.
Persistent contrails are of interest to scientists because they increase the cloudiness of the atmosphere.[SUP]
[1][/SUP] The resulting cloud forms may resemble cirrus, cirrocumulus, or cirrostratus, and are sometimes called
cirrus aviaticus.
Persistent spreading contrails are thought by some, without overwhelming scientific proof, to have a significant effect on global climate