Contrails (/ˈkɒntreɪlz/; short for "condensation trails") or vapor trails are long, thin artificial (man-made) clouds that sometimes form behind aircraft. Their formation is most often triggered by the water vapor in the exhaust of aircraft engines, but can also be triggered by the changes in air pressure in wingtip vortices or in the air over the entire wing surface. Contrails are made of water in the form of a suspension of billions of liquid droplets or ice crystals.Depending on the temperature and humidity at the altitude the contrails form, they may be visible for only a few seconds or minutes, or may persist for hours and spread to be several miles wide. The resulting cloud forms may resemble cirrus, cirrocumulus, or cirrostratus, and are sometimes called cirrus aviaticus. Persistent spreading contrails are thought by some, without overwhelming scientific proof, to have a significant effect on global climate. According to the FAA, in conjunction with scientific experts at the EPA, NASA and NOAA, "Contrail cloudiness might contribute to human-induced climate change. Climate change may have important impacts on public health and environmental protection .... Changes in cloudiness resulting from human activities are important because they might contribute to long-term changes in the Earth’s climate. Contrails’ possible climate effects are one component of aviation’s expected overall climate effect."