On Mon, 23 Nov 2015 11:07:26 -0000, "Bernard Burton"
wrote:

"Eskimo Will" wrote in message
...

"Norman" wrote in message
...
David Mitchell wrote:

A Norwich to Aberdeen flight is just overhead at 36000 feet with a
contrail
clearly below the cloud cover. Is this an apparition? And am I really
unobservant, because it's something I don't recall seeing before?

It's not uncommon to see contrails below a cirrus layer here. Also, they
are
often embedded in the cirrus. From time to time very persistent contrails
spread out and merge to give almost a complete cover of cirrus. Perhaps
there
should be a new cloude type - 'cirrus jumbogenitus' :-)


Don't see many contrails here as not on main flight paths but I have seen
such phenomena frequently elsewhere. Due to exhaust gases having different
condensation temperature to water I think?

Will
--
http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm
Will Hand (Haytor, Devon, 1017 feet asl)
---------------------------------------------

When there is a supersaturated wrt water layer at temperatures below zero,
and especially for temperatures below -20C, the addition of water plus
particulate nuclei derived from the combustion of hydrocarbon fuel is
generally sufficient to produce a cloud (contrail) at pressures at
medium/high tropospheric levels. I would guess that in such a layer,
condensation due to the reduction of pressure caused by the passage of air
over the aircraft will in itself be sufficient to produce a trail. It
should also be noted that the downwash from the passage of an aircraft can
push exhaust products down into a saturated layer even though the aircraft
may be flying above the layer. In this respect, it is possible for an
aircraft flying just above a thin layer of cloud to produce both a distrail
and a contrail in that layer. Saturated layers can be very thin vertically,
in the region of tens of metres, at least initially, but can thicken after
cloud forms due to vertical mixing caused by radiational cooling of the
cloud top. Thin layers can reach saturation at the crests of atmospheric
waves either orographically or dynamically caused, and several such layers
can exist in the same region simultaneously.

Thanks for that very informative reply.

Jeff

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