Lesson 1 of 5: What Are Contrails?
Contrails (short for "condensation trails") are line-shaped clouds that form behind aircraft flying at high altitudes. They're made of ice crystals—the same material that makes up natural cirrus clouds.
When you see a white trail stretching across the sky behind an airplane, you're watching water vapor from the engine exhaust freeze almost instantly in the extremely cold air at cruising altitude.