[cesaref]

I believe contrail formation requires nucleation sites, which is normally provided by soot particles from the burning fuel.

[labster]

It doesn’t in a really moist layer. Ice condensation nuclei are relatively rare (thus the silver nitride seeding idea), so a lot of times ice clouds form when a parcel of air gets supersaturated. This is pretty common.

[kwhitefoot]

Good point, thanks.