[RobRoy]

>>Oh. And you see how this example contrail is lighted on its bottom side. Very bright. Why is that ?<<

I found your question.

Simple question. Simple answer: The sun.

The aircraft is high enough in the atmosphere to still be in sunlight. It is why the cloud is lit up as well.

BTW, that looks like a missile. Notice the “corkscrew” pattern?

JK. :)

[justa-hairyape]

Correct. Simple answer. The sun was below the horizon. The sun was illuminating the bottom of those contrails high up in the sky. Lighting. Very simple stuff. Now, why is the bottom of this plume/contrail dark ? You state it was a plane at high altitude flying to Phoenix during sunset.