Definition
A visible trail of condensed water vapor or ice crystals that forms behind an aircraft flying at high altitude. It occurs when hot, moist exhaust gases mix with cold, low-pressure air, causing the water vapor to condense or freeze into a streak of cloud that follows the aircraft. Also called a contrail (short for condensation trail).
Plain English
The white line you sometimes see in the sky behind a high-flying jet. It is a thin cloud made when hot, damp engine exhaust meets very cold air and the moisture freezes into tiny ice crystals.
Context Anchor
Seen behind aircraft in flight, especially jets operating high enough for the surrounding air to be very cold.
Derivation
From 'vapor' (Latin vapor, meaning steam or moisture) and 'trail' (a track left behind). The word describes exactly what it is: a trail made of vapor. The alternate name 'contrail' is a contraction of 'condensation trail.'
Why Pilots Care
Vapor trails reveal an aircraft's position and altitude visually, which matters for traffic awareness, military operations, and weather observation. Their presence or absence also indicates the moisture content and temperature of the air at altitude.
Analogy
It is similar to seeing your breath on a cold morning: moisture becomes visible because the air is cold enough to turn it into tiny droplets.
Grounding Statement
Picture a jet crossing very cold, moist air; a white line appears behind it because moisture has turned visible in the cold air.
Intuition Check
Do not assume a vapor trail is smoke. A vapor trail is made of water droplets or ice crystals, not burning material.
Example Sentence 1
Climbing through 32,000 feet, the airliner began leaving a long vapor trail across the clear morning sky.
Example Sentence 2
Persistent vapor trails helped the pilot spot other traffic during the visual approach.