Contrails form when hot humid air from jet exhaust mixes with environmental air made of low pressure and temperature, similar to what happens in cloud formation.
desert trivia: Jetstream, different from jet trails, is a flow of wind that blows horizontally through the upper layers of the troposphere, usually west to east, at an altitude of about seven miles. A jetstream develops where air masses of different temperatures meet. The greater the temperature difference, the faster the wind velocity inside the jet stream, sometimes reaching up to 200 miles per hour. Jetstreams can be hundreds of miles wide, thousands of miles long, and over a mile thick.
photo: Joshua Tree National Park, 2000