I have reasons to be suspicious of the phenomenon
It seems illogical that each molecule of exhaust water can attract to itself ten thousand atmospheric water vapor molecules out of its surrounding air, but that is in fact what is happening. The factor can be greater than that, depending on the degree of supersaturation and the distance and time the agglomerating ice crystals spend falling through supersaturated conditions. This changes the trail's mass from
pounds to
tons.
It's possible for a
single trans-USA jumbo flight to put 80,000 tons of fine white ice crystals into the skies. Possible, but unlikely, because supersaturated conditions rarely extend continuously over such a three-thousand-mile distance. They do, however cover two-hundred-mile distances quite frequently (about 17% of the time) which means that to affected onlookers the contrail would appear horizon-to-horizon.
This ten-thousandfold amplification of effect is counter-intuitive. The link's somewhere in Contrailscience and on the first page of jazzroc.wordpress.com
Regions of supersaturation in the earth's atmosphere seem to be associated with
areas of relative stillness of the upper troposphere and lower stratosphere, where water vapor molecules, relatively unaffected by gravity, have had time to migrate
upward from the relatively wet and turbulent lower troposphere by diffusion alone.
The general reduction in motion energy (and extended period of time) allow a greater dynamic packing of (ever-moving) water vapor molecules into the (ever-changing) spaces between (ever-moving) atmospheric gas molecules, in a form of cosmic billiards*. That's supersaturation. If you want the maths behind it then Ross Marsden's the guy you need. I think.
* In this case, sound waves impart (temporarily) energy to change all collision circumstances. Then they remove the energy as well. At 1:53.