Contrails
26,000 - 40,000 ft (8,000 - 12,000 m)
“Lines drawn across the sky, departures we cannot see returning”— Contemporary observation
Contrails—condensation trails—are the lines aircraft draw across the sky, temporary signatures of human presence in the upper atmosphere. They form when hot, humid exhaust from jet engines mixes with cold air, causing water vapour to condense and freeze into ice crystals.
Some contrails vanish almost as quickly as they form, disappearing within seconds behind the aircraft. Others persist and spread, sometimes evolving into cirrus-like sheets that can cover large areas of sky. The difference depends on the humidity of the air the plane is flying through.
There's something oddly poetic about contrails. They're evidence of journeys in progress, each white line representing hundreds of people hurtling through the sky toward destinations near and far. And they're temporary by nature—here for a moment, then gradually dissolving back into the blue.
things worth knowing
- Contrails can spread to cover 20,000 square kilometres in a single day
- The Allies used contrails to track German aircraft in World War II
- Persistent contrails only form when humidity is above about 60% at flight altitude
- On September 11-13, 2001, scientists studied the temperature effects when US airspace was closed and contrails disappeared
weather wisdom
Persistent contrails indicate high humidity aloft, potentially predicting weather changes