What is a contrail?
A contrail is a cloud made by an aircraft. Jet engines put out hot, humid exhaust, and when it meets very cold air high up, usually colder than about minus 40 degrees, the water vapor freezes into ice crystals. Whether the trail vanishes or lingers depends on how humid the air is at that height.
What it’s made of
The name is short for condensation trail. Burning jet fuel produces water vapor, more than a kilogram of it for every kilogram of fuel, along with tiny soot particles. At cruising altitude the surrounding air is extremely cold. The hot, moist exhaust cools in a fraction of a second, the water condenses onto the particles and freezes, and a line of ice crystals forms behind the aircraft. That’s a contrail: a thin, artificial cloud.
Why some planes leave them and others don’t
Two things decide it: temperature and humidity. Contrails generally need air colder than about minus 40 degrees (the same in Celsius and Fahrenheit), which usually means above roughly 26,000 feet. Below that, the exhaust is too warm to freeze, so low planes, turboprops and small aircraft rarely leave one.
Humidity varies in layers. Two airliners can cross the same patch of sky a few thousand feet apart, one drawing a long white line and the other leaving nothing, because one is flying through moist air and the other through dry.
Short, long and spreading
- Short-lived: in dry air the ice crystals turn back into vapor within seconds or minutes. The trail stays short behind the plane, like a tail.
- Persistent: when the air is already close to saturated with ice, the crystals don’t evaporate. The trail stays long after the plane is gone.
- Spreading: in very humid air, persistent trails widen and blur into sheets of thin cirrus cloud. On busy flight routes, a clear morning can turn hazy this way.
A sky full of long-lasting trails tells you the upper air is moist, which often happens ahead of an approaching weather system.
A contrail means the plane is high
Because they need such cold air, contrails are a good altitude clue: a plane drawing one is almost always above about 25,000 feet, and most often at airline cruising height. How high different aircraft fly. The exception is on humid days near airports, when you may see brief wisps curling off the wingtips or flaps of a plane coming in to land. That comes from the drop in air pressure over the wing, not from the exhaust, and it’s gone in an instant.
Count the engines
Each engine makes its own trail, at least for the first few seconds before they merge. Two lines usually means a twinjet like an A320 or 777. Four lines means four engines: a 747, an A380 or a military jet like the KC-135. Eight engines in four pairs, if you’re very lucky, is a B-52. Which planes still have four engines.
What about “chemtrails”?
The long-lasting trails some people point to are persistent contrails, behaving the way the physics says they should in humid air. In a 2016 survey of 77 atmospheric chemists and geochemists, 76 said they had seen no evidence of a secret spraying program.
Who’s drawing that line?
The plane is at the leading end of the trail, often a tiny glint in the sunlight. Find the blip in that direction on a live aircraft map and tap it to see what it is, where it’s going and how high it’s flying. Here’s the quick way.
