Most likely the contrails are left by high flying jet aircraft. Many of the ones we have now are extremely fast and generate a lot of energy. If you are considering that these may have been left by U.F.O craft, perhaps from another world, I can't remember an incident where any left residual contrails. Most of the air craft that we have in service or may be experimental, is well noted and in plain sight, although I suspect there are some that we may not be aware of, and no one wants us to be.
You can help the HubPages community highlight top quality content by ranking this answer up or down.
well i rarely see contrails around where i live (los angeles). But i do indeed see chemtrails, which do not dissipate. I recently saw this which really opened up a whole dimension of thought that i had never, considered. maybe you guys would enjoy reading this as well.