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Contrail

What Is a Contrail?

· 3 min read

A contrail is the thin white line a plane leaves behind at cruising altitude — condensed water vapour from the engines freezing into ice crystals. Here's how they form and what they tell you.

A contrail — short for condensation trail — is the thin white line a plane draws across the sky at high altitude. It looks like smoke, but it is really just frozen water: vapour from the engine exhaust condensing and turning to ice in the cold air behind the aircraft.

How a contrail forms

Jet fuel is a hydrocarbon, and burning it produces water vapour along with hot exhaust gases. As that hot, humid exhaust leaves the engine and mixes with the extremely cold air at cruising altitude — often colder than −40°C — the water vapour condenses into tiny droplets and freezes almost instantly into ice crystals. Those crystals scatter sunlight, which is why the trail appears bright white.

It is the same physics as seeing your breath on a cold morning, just happening at 10,000 m and moving at hundreds of knots.

Why some flights leave long trails and others leave none

Whether a contrail forms — and how long it lasts — depends almost entirely on the air the plane is flying through:

  1. Cold enough — the surrounding air must be very cold, which is why contrails appear at high cruise levels but not near the ground.
  2. Humid enough — if the upper air is already close to saturated, ice crystals persist and the trail can stretch for miles and spread into cirrus-like sheets.
  3. Dry air — if the air is dry, the crystals sublimate (turn straight back to vapour) within seconds, and the plane leaves a short trail or none at all.

This is why you can watch two aircraft at a similar height where one draws a long white streak and the other leaves a clear sky — they are simply passing through pockets of air with different humidity.

What you’ll see on the radar

Contrails form at the same high cruise levels you’ll see on the FlyOverhead radar — typically FL350–FL400, the band where the air is coldest. A jet leaving a long, persistent trail overhead is almost always at cruise, not climbing or descending.

If you match a trail overhead to a target on the radar, the altitude readout will usually confirm it: a plane low enough to be on approach rarely leaves a contrail, because the warmer air down low never lets the vapour freeze.

Frequently asked

What is a contrail made of?

A contrail is made of tiny ice crystals. Jet engine exhaust contains water vapour, and at the very cold temperatures of cruising altitude that vapour condenses and freezes almost instantly into a visible white line.

Why do some planes leave contrails and others don't?

Contrails only form when the air is cold and humid enough — usually above about 8,000 m. A plane crossing drier or warmer air at the same height may leave no trail at all, so two aircraft nearby can look completely different.

Are contrails the same as chemtrails?

Yes, in the sense that every trail you see is a contrail — a condensation trail of ice. The 'chemtrail' idea has no scientific basis; the trails are ordinary frozen water vapour, and how long they last simply depends on how humid the air is.