Michael Mcneil wrote:
"Edmund Lewis" wrote in message
ups.com

I've noticed at least 3 types of contrail from the ground. One the
"needle" you mention, another a longer, wider version of it where
you
can see the division in the middle, this persists and widens for
several minutes then vanishes, and one that refuses to dissipate,
just
spreads out and ends up turning into what looks like cirrostratus.
I've
seen a lot of the latter type just lately- this is the type I think
might be meddling with the weather (though I shan't go into that
here).

Quite common on the approach route to Manchester Airport over a
region
that is noted for its dry (by UK standards) micro-climate.

The region is a 6 mile triangle of flats once marsh sea-shore
surrounded
by low hills and is given to mists.

On sunny days these trails will last forever it seems.

When an aircraft passes through air that is nearly saturated the
vortices of the air it displaces must be equal to the mass of the
craft.
Complex aerodynamics reduced to simple physics is that the air
supports
the wings and the wings support the plane.

The pressure changes alone must be enough to cause condensation but
as
with the vortices themselves the process is adiabatic or self
contained.
If the weather was going to cloud over anyway then these things would
have formed anyway.

However some bean counters have suggested that the overall picture is
that aircraft take out the high and low points.

What that ammount to is that while overall temperature change is
small
there in less movement in the thermometer.

Or not as the case may be.

At least that is what I gather from the study of the temperatures
over
places like New York after the attack shut down all the airports in
the
USA.

What that study showed was that the diurnal range (difference between
max and min) increased by at least 1 deg C when the contrails were
virtually non-existent*. It wasn't just New York but all over much of
the USA IIRC.

*They picked up the odd contrail from airforce jets and studied how
they spread as well.

http://www.theozonehole.com/airtraffic.htm
http://wired-vig.wired.com/news/tech...,52512,00.html
http://www.climateark.org/articles/r...p?linkid=14028

Edmund