To answer this question, lets first identify what a contrail is. A contrail is the con densation trail that is left behind by a passing jet plane. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing occurs directly behind the plane due to the turbulence generated by the engine.
Water is a normal byproduct of combustion in engines.
The length of time that a contrail lasts is directly proportional to the amount of humidity that is already in the atmosphere. However, if the atmosphere is too dry, no contrails will form. Occasionally a jet plane, especially if ascending or descending, will pass through a much drier or more moist layer of atmosphere which may result in a broken pattern to the contrail, with it appearing in segments rather than in one continuous plume. Contrails can be found over most of the planet. Now that jet plane traffic, both civilian and military, can be at anyplace over the globe at anytime, contrails are becoming more and more common.