[SeeSac]

Also, given the heavy air traffic in the area, if atmospheric conditions are the cause, why aren't there other contrails that look similar to this one?

Answered 10,000x in other threads. Most of that traffic was not high enough to produce contrails.

[TigersEye]

Answered 10,000x in other threads. Most of that traffic was not high enough to produce contrails.

That is absolute baloney. You can go to Airport Monitor-Bob Hope Airport and run the air traffic patterns over LA for Nov 8th between 5:00 PM and 6:00 PM and see literally dozens of flights over 39,000 feet. There are at least six in the air at that altitude or higher at any given time.