In article ,
Bill wrote:

On Mon, 24 Oct 2005 17:36:15 GMT, Larry Dighera
wrote:

On Mon, 24 Oct 2005 12:25:15 -0500, Bill wrote in
::

It was a tube of swirling distortion,
dropping down off his wingtip and then coming back to us.

Did what you saw have any moisture component that made it more
visible?

No. We discussed this as well. This was nothing like the vortices
you see when it's humid, especially from jets on landing.

This was just a visual distortion, like what would happen if you
looked through glass that had a big dimple in it. And if you weren't
almost perfectly centered on it, it was invisible.

You could not see it from the side of the plane, nor from behind it
unless the center of the vortex was actually hitting the windshield.

The index of refraction of air changes with density. Light moving
between regions of different density gets bent by the changes in the
IofR. You perceive this bending as distortion. This is what causes the
"shimmering" effect on a hot day. Light moves from hot, less dense air
near the ground to cooler, more dense air by your eyes and is refracted
at the boundary between the two air masses. The shimmering occurs
because the hot air rises, which causes the temperature and hence the
density of the air to fluctuate.

The effect is most pronounced where there is a net difference in density
between the air around the observer and the object being perceived. In
this case the light passes through only one refraction boundary. The
light is bent only once, and so it arrives at the observer at a
different angle than it left the object being observed, making it look
as if the object has moved quite a bit from where it really is. (You
can see this effect very dramatically in a fish tank.)

If the density at the object and observer are the same you can still
perceive refraction if there is more/less dense air in between, but the
effect is much less. This is because the light passes through two
refraction boundaries. The first one bends the light one way, and the
second one will (typically -- it depends on the geometry) bend it back,
so the net effect is not a change in the angle of the light but simply
an offset. The magnitude of the offset depends on how far the light
travels in the different density region. Because vortices are narrow
the refraction offset viewed from the side is too small to be perceived
by the unaided eye. But inside the vortex the light passes through only
one refraction boundary so you get an angular offset that you can
perceive. The "shimmering" is caused by small changes in the geometry
of the boundary of the vortex as the plane moves through the air.

rg