An aircraft contrail is a mixing cloud caused by the emission of hot gases and water vapour into the atmosphere by jet engines. In this case the contrail is casting a shadow onto cirrus clouds below. In order to reinforce something important that is widely misunderstood by some people, there are no chemtrails. Every contrail that you see is indeed artificially created but the process is not sinister or mysterious.
Time-Lapse Movies of Clouds
There a number of movies (in Quicktime and Windows Media Format) available on our Time-Lapse Movie page.