Definition
Visible trails of condensed water vapor that form behind an aircraft when hot, moisture-laden engine exhaust mixes with cold, low-pressure air at high altitude. The water vapor in the exhaust cools rapidly and condenses, then often freezes, creating a long white cloud-like streak behind the aircraft.
Plain English
The white streaks you see trailing behind a high-flying aircraft. They form when hot, wet engine exhaust hits very cold high-altitude air, causing the water in the exhaust to instantly turn into a line of tiny ice crystals.
Context Anchor
Seen in weather discussions, high-altitude flight, and visual observation of aircraft operating in cold upper air.
Derivation
From Latin 'con-' (together) and 'trail.' Short for 'condensation trail' — a trail formed by condensation. The 'exhaust' part specifies that the moisture comes from the engine exhaust itself, distinguishing it from contrails formed by other causes.
Why Pilots Care
Contrails reveal an aircraft's position and track to anyone looking up, which matters for military operations and is sometimes useful for spotting traffic. They can also indicate the temperature and humidity conditions at altitude.
Analogy
Like seeing your breath on a cold morning — warm, moist air from your lungs hits cold outside air and the water vapor becomes briefly visible. An exhaust contrail is the same thing, just at 30,000 feet and on a much larger scale.
Grounding Statement
Picture a jet crossing very cold air and leaving a white frozen breath behind it.
Intuition Check
Exhaust contrails are not smoke from an engine problem. They are usually frozen water from normal engine exhaust mixing with very cold air.
Example Sentence 1
At cruise altitude the airliner began producing thick exhaust contrails visible from the ground.
Example Sentence 2
The student pilot learned to recognize exhaust contrails as a sign of cold, humid air at altitude.