The combination of high humidity and low temperatures that often exists at aircraft cruise altitudes allows the formation of contrails. Contrails have been a normal effect of aviation since its earliest days. Depending on the temperature and the amount of moisture in the air at the aircraft altitude, contrails can either evaporate quickly or they can persist and grow.
Engine exhaust produces only a small portion of the water that forms ice in persistent contrails.
Persistent contrails are mainly composed of water naturally present along the aircraft flight path. Of these emittants, only water vapor is necessary for contrail formation. Sulfur gases are also of potential interest because they lead to the formation of small particles. Particles suitable for water droplet formation are necessary for contrail formation. Initial contrail particles, however, can either be already present in the atmosphere or formed in the exhaust gas.