A four-engined jet, sometimes called a quadjet, is a jet aircraft powered by four engines. The presence of four engines offers increased power, allowing such aircraft to be used as airliners, freighters, and military aircraft. The white lines behind the plane are referred to as contrails - an abbreviation for condensation trails. Water vapor is one of the byproducts of jet fuel combustion and will turn into ice crystals in the cold air at the high elevations where jet airplanes fly. Those ice crystals create a cloud (the contrail), which does not pose any public health risk. Contrails can evaporate quickly if humidity is low, or they can linger, and even grow by taking in water from the surrounding atmosphere, when humidity is high.
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