I figure this is more of an ops question so I am posting it here.
A couple of days ago early in the morning, during gym class, I saw an American 727 fly over, close enough that I could identify it. The location where I saw it is about 35 miles NW of PHL. In the 13 years I have been living here, I have never seen a commercial jet fly low enough in this area that I could identify it.
Similarly, after school, while walking home, I saw two sets of contrails. I could barely make the type (737) on one, but no scheme, and nothing on the other as they were both directly overhead.
My questions are: Why was the 727 so low (it wasn't terrain following, but it was low), and how high do planes need tp be to produce contrails?
Thanks for your time and answers.