Last Friday evening, we had quite a few people comment about what appeared to be "sky tornadoes" or "snakes in the sky" above a cloud bank to the west of Seattle. To many, it looked like perhaps a vortex coming out the top of the clouds.
But I think it was just an optical illusion -- a combination of air traffic, relative distance, and the dim lighting of the dusk sky.
Those "snakes" are likely just jet contrails as the jet flew off into the distance (or approached from the distance) and the contrail just runs into the cloud. Had this occurred earlier in the day when the sky was brighter, we likely would have noticed the different coloration between the two objects, but since it was dusk, it became more of a monochrome, two dimensional effect.
As to why the contrail just ends suddenly, it's likely there was a boundary where conditions changed in the atmosphere to where the contrails dissipated quickly, versus dispersing much more gradually and thus giving this "snake" appearance.
(Note: I did use Photoshop to just add some brightness to the photos so you can better see the effect. Here is the orignal version, unbrightened)