First of all, sorry to post a topic and then not reply for a week;
All part of earning a living. Hope you understand.

I thought I had replied but I guess it went into the Ether somewhere.
IIRC, I was going to say that it seems there are different "types" of
contrails. Three come to mind:

1. I call Diffuse Contrail. Imagining the sky as the Celestial Sphere
of Astronomy, As the jet crosses the diameter ( or some chord) of the
sky it leaves a long contrail that persists until well after the jet
is long gone. This same contrail then slowly diffuses laterally
(outward) and forms the "Chem-trail" of Conspiracy Theory.

2. I call the Dynamic Contrail. It forms a certain length behind the
jet and then "moves" across the sky with the jet. The contrail doesn't
lengthen, rather it follows the jet across the sky.

3. I call the Hanging Contrail. The jet that formed it ( and most of
the rest of the original contrail) are long gone. But a "piece" or
chunk still hangs up there, apparently not moving. Caught at dusk,
illuminated at high altitude by a sun that's already set for you the
ground observer - these can be very brilliant and dramatic.

IMO, it's number 3 that accounts for  *some" dusk UFO sightings.
I recall that this photo showed bright highlights at both ends of the
trail. This something I'd associate with type #3. Type #1 usually has
no brilliancy and #2 type has it only on the leading edge.

Why these different types forms is a matter of alltitude and weather
I guess - not sure. Nor do I know if this matters a hill of beans !
But there you go, my two cents.

                  Thanks for Replying,

                                               Andrew