I'm surprised nobody has figured this one out yet!
Has anyone seen a high flying jet from the ground leaving no vapour trail? What is another name for vapour trail? With reference to the rainbow sightings, what is essential for a rainbow to be seen?
The answer is: water vapour. A "vapour" trail is formed by condensate on particulate matter from the engines. Clouds form when the moisture in the air has something upon which to condense, OR if the moisture in the air is cooled enough (dewpoint) to produce condensation (clouds). Exhaust from an engine is similar to the idea of "cloud seeding" - when aircraft dispense that iodide mixture to precipitate the moisture in the air, usually to accelerate rainfall and preclude the occurence of violent thunderstorms (if memory serves me correctly).
Normally the higher altitudes are quite dry, but in the advance of a warm front, moisture occurs in varying degrees at those altitudes. When I asked earlier about planes leaving no condensation trails, there are times when they leaves a short trail that sublimates almost immediately - a sign that there is some moisture in the air; sometimes the trail lasts for an hour - a sign that there is a lot of moisture in the air.
So like the appearance of high cirrus cloud advents the approach of bad weather, so might the appearance of long standing condensation trails.
Waddya'all think about that?!
Note of interest: While in the arctic, it was a common occurence to see the turboprops landing with a stream of "smoke" coming from their exhausts. Our company got calls on more than one occasion suggesting an airplane was on fire! It was just so cold and damp that a condensation trail occured.
Note of interest #2: about 6 months ago, I was returning from Hamburg to Toronto at FL390, when out of the corner of my eye, I saw a condensation trail above us, but quite high, and moving a lot faster than our M0.80! Sure enough, BA1 checked in with Gander OCA as we watched this "speck" (we could not see the actual plane at the head of the trail) continue its trek at FL585/M2.0. A week later, in virtually the same spot, the same occurence, but just the radio call - no contrail. After talking with other pilots who have done the route many more times than I, this was a rare sighting.
Great question, by the way.