Contrails—short for condensation trails—are the white streaks commonly seen behind airplanes flying at high altitudes. They form when hot, moist exhaust from aircraft engines meets the cold, low-pressure air of the upper atmosphere, causing the water vapor to condense and freeze into tiny ice crystals. These crystals create the visible trails. Depending on atmospheric conditions such as humidity and temperature, contrails may vanish quickly or linger, spreading out to resemble thin, wispy cirrus