Contrails or condensation trails

  • Height of base: above 20,000 ft. 
  • Latin: No formal latin classification – but the word condensation comes from the latin word “condensatione”. 
  • Precipitation: None

What are contrails? 

Contrails are long thin lines of cloud, usually seen behind an aircraft.

How do contrails form? 

Aeroplane jet engines produce water vapour as a bi-product of burning fuel. Above 20,000 feet, the air surrounding the aircraft is well below freezing so it cools down the water vapour coming out of the back of the engines. This causes the water vapour to condense rapidly, and then freeze. Tiny particles from the engine, known as condensation nuclei, act as a starting point for condensation to take place, leaving thin trails of ice crystals behind the aeroplane's engines.

What happens next depends on how dry or how humid the air is. If the air is very dry, the ice crystals will sublime (change phase directly from solid to gas) and become invisible. If the air is humid, the water droplets or ice crystals will stay where they are, often spreading out, leaving a fluffy trail where the aircraft has passed. Trails may last for many hours leaving the sky criss-crossed with lines, and mixing with cirrus cloud.

What weather is associated with contrails?

Contrails are not large enough to cause any weather on the ground. We tend to observe them in empty skies in high pressure situations when there are very few other clouds around.

How do we categorise contrails?

Strictly speaking, there is only one type of contrail, however, there is a similar effect called “Wingtip vortices”.  As an aeroplane wing generates lift, it causes a vortex to form at the wingtip and at the tip of the flaps. These wingtip vortices persist in the atmosphere after the aircraft has passed. The reduction in pressure and temperature across each vortex can cause water to condense, producing a thin line of water droplets that looks just like a contrail. This effect is more common on humid days so wingtip vortices can sometimes be seen behind the wing flaps of airliners during takeoff and landing. Unlike contrails, wingtip vortices are usually only seen at low altitude where the aircraft is travelling slowly after takeoff or before landing. They trail behind the wingtips and wing flaps rather than behind the engines, and they evaporate quickly just a few metres behind the aircraft.

Where an aircraft passes through a cloud, it can disperse the cloud in its path. This is known as a distrail (short for "dissipation trail"). The plane's warm engine exhaust causes existing water droplets in pre-existing clouds to evaporate, leaving a clear wake through an otherwise cloudy sky.

Another related but different effect is "ship trails". These are clouds that form around the exhaust released by ships into still but humid air just above the ocean. Water vapour already in the air condenses on the aerosol particles in the exhaust. In the case of ship tracks, only a little of the condensing water vapour comes from the ship’s engines. Most is already present in the air before the ship arrives. The resulting clouds resemble long strings over the ocean. These are often much wider than contrails so they can often be seen on satellite pictures of the earth.