Contrails, short for "condensation trails," are visible cloud-like trails that form behind aircraft as a result of the water vapor and other emissions released by their engines. When aircraft engines burn fuel, they produce water vapor and exhaust gases, including carbon dioxide, nitrogen oxides, sulfur compounds, and small particles. In certain atmospheric conditions, typically at high altitudes where the air is cold and humid, the water vapor in the engine exhaust condenses into tiny ice crystals or water droplets, forming contrails. Contrails have negative implications primarily in terms of their impact on climate. When contrails form, they can contribute to the formation of cirrus clouds, which have a warming effect on the atmosphere.