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Speaker 1: Welcome to brain Stuff from how Stuff Works dot com

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Speaker 1: where smart Happens. Hi, I'm Marshall Brain with today's question,

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Speaker 1: why do those long white clouds formed behind jets flying

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Speaker 1: high overhead? The clouds that jets form on certain days

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Speaker 1: are called condensation trails, or contrails. Some days the contrails

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Speaker 1: will form thin lines that cross the entire sky, other

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Speaker 1: days they will be much shorter, and on some days

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Speaker 1: there won't be any at all. A contrail forms because

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Speaker 1: one of the components of jet engine exhaust is water.

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Speaker 1: Jet fuel is made of carbon and hydrogen. When jet

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Speaker 1: fuel burns with oxygen, most of the exhaust consists of

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Speaker 1: CO two or carbon dioxide, and H two O or water.

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Speaker 1: A big jet burns hundreds of gallons of fuel per hour,

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Speaker 1: meaning it produces hundreds of gallons of water. The water

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Speaker 1: is generally an invisible vapor. When you exhale, your breath

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Speaker 1: contains a great deal of invisible water vapor as well.

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Speaker 1: You may have noticed that on certain days in the winter,

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Speaker 1: your breath will form a cloud of condensation when you exhale.

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Speaker 1: In the summer, however, you don't see your breath. You

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Speaker 1: see the condensation in winter because cold air can hold

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Speaker 1: a lot less moisture than warm air, so in the winter,

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Speaker 1: when the moisture in your breath hits the cold air,

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Speaker 1: the moisture condenses into a visible cloud. The same thing

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Speaker 1: happens when a jet engine exhales. If the temperature, winds,

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Speaker 1: and humidity in the upper atmosphere are right, long white

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Speaker 1: trails form when the moisture in the exhaust condenses. For

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Speaker 1: more on this and thousands of other topics, does that

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