[thedoc (OP)]

Sitting in our kitchen, looking at aeroplanes out of the window I noticed a different colour in front of the plane, as the plane moved along through some wispy cloud.  As it crossed the sky there was a section in front of it that was darker.  It almost looked as though the cloud was dispersing well in front of the plane as the plane went along.  I wondered whether there’s any way that the plane itself could be causing the cloud in front of it to disperse quite a distance in front of it, almost like the cloud is expecting it to arrive!

(Barry also said that the Sun was to the left, and relatively low in the sky)
Asked by Barry, Nottingham

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[thedoc (OP)]

We think that the sun is behind the plane and casting a shadow of the contrail.  As the sun is behind the plane the shadow appears in front of the plane, as it were.  The cloud is so wispy and tenuous the only way you can see it is because the sun’s lighting it up.  Once you’re in shadow you can’t see the cloud so it looks like the cloud’s disappeared.  Actually, if you look very closely above the contrail you can see a little bit of a shadow following above the contrail as well. I think what it is, it’s just a shadow cast from an interesting direction.

What do you think?

[paul.fr]

Is that the actual picture from the listener, Barry?

Yes, contrails (condensation trail) do like any other cloud cast a shadow. And as you suggest this picture looks like one such shadow, you can also see contrail shaodws due to moonlight.

[paul.fr]

Sort of related.
http://www.atoptics.co.uk/atoptics/contr2.htm

[daveshorts]

Yes it did come from Barry.

[paul.fr]

Sorry Dave, i posted before listening to the podcast.