Some planes leave a trail behind them because of a phenomenon called contrails, or condensation trails. These trails are formed when the water vapor in the aircraft engine exhaust mixes with the very cold air at high altitudes. The water vapor condenses into tiny water droplets that quickly freeze into ice crystals, forming visible white trails in the sky. Whether contrails appear and how long they last depends on the atmospheric conditions, especially temperature and humidity. If the air is humid, the contrails can persist and spread out, sometimes looking like natural cirrus clouds. If the air is dry, the contrails may disappear quickly within a few minutes. Different engines can also affect contrail formation, with more efficient engines producing more water vapor and thus more likely to form contrails.