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A contrail is the condensation trail that is created and left behind when a jet moves through cold air with a low vapor pressure. At high altitudes, temperatures are cold enough that only a small amount of moisture is needed to create condensation. Jet exhaust delivers that moisture in hot, humid air. The turbulence also created by jet exhaust mixes that hot air with the colder air in the atmosphere, resulting condensation. It's the same process that creates a "cloud" when warm breath is exhaled on a cold day.
Contrails were first observed during World War I, but it wasn't until World War II that they became a concern — for two reasons. Firstly, they allowed aircraft to be detected, and secondly, they sometimes impeded visibility so badly, it was impossible for pilots to keep sight of enemy planes during combat. Today, due to increased air traffic over the last half-century, contrails have been observed over nearly every area of the world.
They are made of condensed liquid, so contrails remain in areas of higher humidity longer than they do in drier skies. In fact, they won’t form at all if the atmosphere is too dry. Sailors sometimes watch these patterns to make educated predictions about the weather. If contrails are invisible or disappear quickly, it indicates drier air and stable weather patterns. On days when they persist, higher humidity and lower atmospheric stability are probable, so weather changes may occur.
If contrails linger long enough, they can spread out across the sky, creating what amounts to man-made cloud cover. Scientists and environmentalists share concerns about the effects this may have on the earth's climate, since some estimates report as much as a 20% increase in cloud cover in heavy air traffic areas. This additional cloud cover acts like cirrus clouds, which reflect incoming sunlight and inhibit the planet's heat loss. It can also alter the balance between sunlight and infrared energy in the atmosphere. In the US, the National Aeronautics and Space Administration (NASA) and the Department of Energy (DOE) are sponsoring a research project to study the impact this has on atmospheric chemistry, weather and climate, and additional studies are in progress to examine how jet exhaust affects water vapor content and chemistry of the upper troposphere.
There is speculation that some contrails are part of government conspiracies. It is suggested they are used to "spray" the general population with chemical or biological agents for undisclosed purposes. Governments around the world deny these allegations, and they remain unsubstantiated.