What Are the White Trails Behind Airplanes?

We’ve all seen the long white streaks that airplanes leave behind in the sky. These are called “contrails,” short for condensation trails.

At cruising altitude, temperatures typically hover around –50°C, depending on the season. The airplane’s surface and the exhaust gases from its engines are much warmer than the surrounding air. When the hot exhaust mixes with the frigid atmosphere, the water vapor inside it condenses.

That condensed vapor quickly freezes into tiny ice crystals, forming a visible white trail behind the aircraft. It’s the same principle as the fog that appears when you breathe on a cold window — just happening several kilometers above the ground.