"Many people are familiar with the long streaks of white clouds produced by jets flying at high altitudes. The clouds which are made up of condensed vapor called condensation trails, or contrails, are generally produced by jets flying between 25,000 and 40,000 feet in moist air. If the relative humidity is low, contrails may evaporate rather quickly, however, in high relative humidity conditions, contrails may remain visible for several hours. This indication of moisture content may point to other clouds and, possibly, precipitation moving into the area within the next day or two" (Jeppesen Private Pilot Manual p 6-26)
here you go, hope that helps. They also have a diagram showing that most of the contrail (if not all of it) is actualy made up of ice crystals, not water vapor.
Formerly known as Jap. Srsly. AUSTRALIA: 2 days!