Contrail Shadow

Shadow of a contrail (vapour trail) left by a jet engine in the sky earlier today - looked mad and wasn't there for long!

"Edge Shadow (Volumetric Shadow)
The most interesting type of contrail dark line is when the contrail is lined up with the sun. This produces a slice of shadow through the atmosphere that looks like a dark line when viewed edge-on. It is quite difficult to visualize what is going on since you have to think in three dimensions, and we are accustomed to thinking of shadows as being flat, since they are usually cast on surfaces. You are not seeing a thin dark shadow here, you are actually seeing a huge slab of very faint shadow, but it’s viewed from looking along the edge. Imagine you have a thick sheet of glass. Viewed head on, it’s transparent, but if you look at it from the edge, it seems a lot darker."
(http://contrailscience.com/contrails-dark-lines-chemtrails/)