The contrails in the photo above were generated by an Air Force C-141 Starlifter. Jet fuel is a mixture of hydrocarbons, which burns to produce carbon dioxide and water vapor. The water vapor condenses upon cooling to form small water droplets--the contrail--which is essentially a cloud. To learn more, check out What is a contrail and how does it form? from the National Weather Service. Like clouds, contrails can affect global warming--to find out how, see From Physics Research and Worth a Look.