A contrail is a distinctive streak of cloud formed by a jet engine in certain atmospheric conditions. Contrails tend to form very high up in the atmosphere, and they can linger for hours in some cases, sometimes promoting the development of cirrus clouds. In some communities, there is a great deal of controversy over contrails, with some people believing that they are harmful to the environment, while others believe that their effect is probably negligible.
The term “contrail” is simply a contraction of “condensation trail,” and that's exactly what a contrail is: a trail of condensation created by the interaction between a jet and the environment it passes through. One type of contrail is formed by warm, moist air which passes through the engines of a jet and then comes into contact with cold air, causing the water to precipitate out and condense. Contrails can also be caused by the reduction in air pressure associated with the wings of jets, which causes the air to cool down, promoting condensation and the formation of a contrail.
Classically, contrails appear in the form of large white streaks of cloud in the sky which slowly dissipate. Passing jets can also form what is known as a distrail, where the plane essentially slices through a cloud, leaving a stripe of clear sky behind. Some people say that distrails look sort of as though someone had taken a sponge and wiped a section of the sky clean; many distrails are also associated with contrails, which form inside the distrail.
Because jets are so heavily used around the world, there is some fear that contrails could be having an impact on global climate, and satellite images taken when conditions are ripe for the formation of contrails seem to be supportive of this idea, showing how a network of contrails can develop into cloud cover which would cool the Earth. Some people have also suggested that contrails are laden with chemical pollutants associated with jet aircraft.
While there certainly are cases in which a contrail can join with other contrails and create artificial cirrus clouds, which would obviously impact weather conditions, there are plenty of other cases in which contrails fail to form, or dissipate so quickly that they do not impact the environment. Of far greater concern is the jet fuel used to power these planes, which most certainly does generate pollutants which harm the environment.