Definition
A visible cloud-like trail of condensed water vapor or ice crystals that forms behind an aircraft flying at high altitude. Contrails form when hot, moist exhaust gases from the engines mix with the very cold surrounding air, causing the water vapor to condense and freeze into ice crystals.
Plain English
The white streak you see trailing behind a high-flying jet. It is made of tiny ice crystals that form when the warm, wet exhaust hits the freezing cold air at altitude.
Context Anchor
Seen in weather discussions, high-altitude flight, and visual observation of aircraft leaving white trails across the sky.
Derivation
A shortened form of 'condensation trail.' The word tells you exactly what it is: a trail left behind by condensation.
Why Pilots Care
Contrails reveal aircraft position visually from the ground, which matters in military flying. They can also indicate upper-level humidity, and persistent contrails can suggest moist air that may lead to cirrus cloud formation.
Grounding Statement
Picture a jet crossing a cold upper sky and leaving a white cloud line behind it.
Intuition Check
Do not assume a contrail is smoke from the engine. A contrail is mainly frozen water that becomes visible when exhaust moisture meets very cold air.
Example Sentence 1
Looking up on a clear day, the student pilot could see a long contrail stretching behind a jet at cruising altitude.
Example Sentence 2
In cold moist air the engines produced a persistent contrail that lingered behind the aircraft.