A true contrail consists of ice particles, so they only persist well above the zero degree isotherm. The trail is generated from aircraft exhausts, where water vapour is introduced to the atmosphere. It then cools / condenses very rapidly into ice crystals which reflect light. There is always a gap between the exhaust outlet and the beginning of the contrail, this is where the water vapour is not yet cooled enough to condense.
We used to be given the contrail level in our morning met brief.
I was once told a story by a Group Captain (anyone remmber Phil Langrill?) who told me he went up on an early morning handling check airtest in a Hunter from RAF Valley in a beautifully clear sky. Having climbed on a heading, he levelled off for a short time on the same heading. He then carried out a 180 degree turn, followed by another minute or so straight and level. He then flew a steep turn one way, briefly rolled out, then did another steep 360 in the opposite direction.
Airtest complete, he descended and returned to Valley. Very shortly afterwards, following local complaints about a huge picture of a penis and testicles in the sky, they had to fly a four ship formation to go up and cross it out!!
Last edited by ShyTorque; 29th Apr 2005 at 22:37.