Clouds form at the condensation level. That is, warm moist air rises through the atmosphere, where the air is about two degrees colder for each thousand feet you go up. Once the warm moist air is cooled by this rising action it cannot hold the water vapor as a gas, and it becomes liquid. When a jet blasts through the upper atmosphere it warms the air in a trail behind it (because it's burning jet fuel). Voila, instant cloud. They are called contrails and they are made of water.