Those long white streaks behind airplanes aren’t just “smoke.” They’re called contrails (condensation trails), and under the right conditions, they can spread into cloud layers that last for hours—or even an entire day. Here’s what’s happening: Jet engines release hot exhaust that contains water vapor. At high altitudes, the air is extremely cold (often below -40°C). When that hot, moist exhaust mixes with the cold air, the water vapor rapidly condenses and freezes into tiny ice crystals. If the surrounding air is dry, the contrail disappears quickly. But if the air is humid, those ice crystals stick around—and can even grow as they collect more moisture. That’s when contrails expand and turn into thin, wispy cloud layers that look like natural cirrus clouds. In busy flight corridors, multiple contrails can spread and merge, creating what looks like a hazy “blanket” in the sky. Scientists study this because widespread contrail clouds can slightly affect how heat is trapped in the at...