[Ramius]

Wouldn't an airplane contrail at 30k ft being composed entirely of water vapor and evenly distributed be evenly illuminated from a light source behind it? How could part of it be illuminated and part opaque?

The light isn't coming through from behind the contrail, it is coming at a low angle ~underneath~ and lighting it from below. Just as a setting sun will sometimes light up the undersides of clouds.

[TigersEye]

And yet the furthest west part of the contrail (assuming a jet contrail coming from the horizon) is in shadow and in front of all the clouds in the picture. But you ignored the main point. Why is one edge illuminated and the other edge of the contrail dark? If the light source was from below and as far away as the sun wouldn’t it illuminate the entire contrail evenly?

[Gargantua]

Ramius! You magnificent bastard!!

8^O