Dark below White above ?

[Pace]

Flying up from south France at FL 360 I was telling my senior first officer of a way I was told of telling if other jet traffic was above or below?
Looking in the distance towards the other jet if it jet trail was dark grey the jet is below if it's White it's above.
Sure enough we checked this with the TICAS and quite true!!
What I cannot work out is why?
Looking at other visible moisture the reverse is true!!
Ie look down at cloud below hit by sunlight and the clouds are white while likely to be darker from beneath!
Not the case with jet trails why ? I cannot work it out ?
Regardless it a good tip to know if jets in the distance are below or above you!
Dark trails and they are below! White above! Check next time you fly!
Candy floss pink it is at same level ( joking ; )

Pace

[Kak Klaxon]

What happens if you are inverted?:cool:

[NuName]

How about all the burnt hydrocarbons, they don't exist in the clouds.

[Pace]

NuName

Even with pollution in the trails you would think the sunny side up would be whiter than the gloomier side down but not the case! check it on your next high level flight :) unless i am flying the Citation up side down must get to read the A/H properly :E

Pace

[NuName]

Pace, I see the same as you but I have always thought that the jet exhaust was the reason. I would like to know why also, always wondered but never asked.