The contrail from an airplane is made up of tiny water droplets or ice crystals, just as clouds are. Some of the water is a byproduct of burning fuel, and the rest comes from water vapor condensing from the surrounding air. Once the droplets form, how long they last depends on the temperature and humidity of the surrounding air. With warmer air and/or low humidity, the droplets or crystals quickly evaporate causing the contrail to disappear. On the other hand, colder air and high humidity lets the contrail last much longer, sometimes even growing into larger clouds.