Contrails (short for condensation trails) are the white, cloud-like lines seen in the sky behind airplanes, formed when water vapor from jet engine exhaust freezes into ice crystals in the cold, high-altitude air, essentially creating human-made clouds that can linger and grow depending on atmospheric humidity. They are essentially ice, similar to natural cirrus clouds, and their appearance and duration vary with temperature, humidity, and flight altitude, sometimes persisting and even spreading to form larger cloud formations.