To complete the question under which conditions contrails are forming and under which condition they are longlasting:

"Chemtrails - US Military Continues
To Spray Chemical-Laden Skytrails
By William Thomas
willthomas@telus.net
© 2001 William Thomas
10-20-1

(...)

As NOAA meteorologist Thomas Schlatter explains, for even short-lived condensation trails to form, 'we're talking temperatures lower than about minus 76 degrees Fahrenheit, and humidity at jet altitudes of 70 per cent or more'."
http://www.rense.com/general15/chemusmilitarycontinues.htm

And how can you develop them anyway without the conditions described above:

"According to the National Center for Atmospheric Research in Boulder, Colorado, the only way to form artificial clouds in warm dry air is to introduce enough particulates into the atmosphere to attract and accrete all available moisture into visible vapour. If repeated often enough, the resulting rainless haze can lead to drought."
http://www.rense.com/general15/chemusmilitarycontinues.htm