Why do jets leave a white trail behind them.

On Thu, 19 Jan 2006 16:44:08 GMT, "AliR" wrote:

Hi everyone,

Me and a friend are wondering why jets leave a white trail behind them. My
guess is that it's moisture being compressed as it passes through the
turbine (because some leave a single trail, while others leave two separate
trails). My friend thinks it's because of moisture being cooled as it
passes over the wing (Bernoulli effect).

While flying a bonanza I have seen a small white trail extending from the
wingtip for a little while right after I break through the clouds
(descending).

Is either one of us right?

While I've never seen condensation with my Warrior, I do have some
observations regarding condensation trails or contrails. There are
different types of contrails.

1. Old WWII newsreels showing B-17s flying over Europe. They're
leaving massive contrails from each of those big round engines. I've
read articles saying that during the war they tried to avoid
altitudes/temperatures where that would occur while over enemy
territory since it would make make the aircraft more visible. Just in
case anyone thought this phenomenon was exclusively a jet
characteristic.
http://www.wpafb.af.mil/museum/resea...s/b17f-10a.jpg

2. TV news coverage of the Space Shuttle landing in Florida (where the
air is often more moist than at Edwards). As the shuttle flew admidst
a few cloud buildups just before touching down, a trail of cloud
appeared at each wingtip. Particularly interesting since the shuttle's
engines aren't operating during the landing phase.

3. Scene from the TV show "Black Sheep Squadron" (AKA "Baa Baa Black
Sheep") showing the Corsairs as they made a tight turn. A whisp of
contrail appeared at each wingtip during the high-g maneuver.

4. At an airshow, the B-1 Bomber come to show off its stuff. As the
last part of the demo, it accelerated to just below mach 1 to thrill
the crowd. As it flew along, the entire amidships of the aircraft was
enshrouded in a white cloud during the time the aircraft was operating
at high speed. Didn't leave much of a trail, but impressive phenomenon
all the same.

5. Films of the moon launch with the Saturn V rocket. I've seen this
footage several times, including I think the movie "Apollo 13." As the
rocket accelerates, a ring of cloud appears around the upper end of
the rocket. I've noticed the same thing on TV coverage of Space
Shuttle launches. Again, no trail.

6. Somewhat unrelated. Once while VFR over a smooth undercast over
Asheville, NC (AVL) I saw that nearby Mt. Mitchell was adorned by a
lovely lens cloud on its downwind side. While the air where I was was
smooth as glass, I can only assume that things wouldn't be so pleasant
over near the mountain. Lens clouds are, of course, formed by the
pressure changes due to high winds over the mountain, similar to some
of the aerodynamic contrails I've noted above. (BTW, in case anyone is
wondering, the undercast cleared out as promised by FSS at my
destination of TRI.)

Just a few examples I remember seeing. Hopefully, someone else can
expand on this interesting subject.

RK Henry