Defence Aircraft Noise

Condensation Trails

Condensation trails or ‘contrails’ are white, cloud-like streams that are sometimes visible behind jet aircraft, normally when cruising at high altitude. Contrails are emitted by jet aircraft under certain conditions. Jet aircraft emit a range of gasses in flight, including water vapour and sulphur particles.

At the high altitudes at which jet aircraft cruise (26 000 ft or 8 000 m), the air is very cold. Colder air is less able to hold water vapour. Therefore, if there is a large amount of water vapour in the air (high humidity), it rapidly condenses on sulphur particles that have come out of the aircraft engine, turning into droplets of water. Water vapour emitted by the aircraft engine will also turn into droplets of water in the same way.

At such high altitudes the temperature is far below freezing and the droplets will quickly form ice particles, making up a white contrail. Being made from ice, contrails are harmless to health. Eventually, when conditions become dry enough (lower humidity levels), the ice particles will evaporate, meaning that they do not reach the ground.

Find more information on contrails by visiting Airservices Australia website.