[TigersEye]

“On the con-trail, however, the illumination from the sun appears on the right-hand-edge”....

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?

[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.