Speaker 1: Welcome to brain Stuff from how Stuff Works dot com
Speaker 1: where smart Happens. Hi, I'm Marshall Brain with today's question,
Speaker 1: why do those long white clouds formed behind jets flying
Speaker 1: high overhead? The clouds that jets form on certain days
Speaker 1: are called condensation trails, or contrails. Some days the contrails
Speaker 1: will form thin lines that cross the entire sky, other
Speaker 1: days they will be much shorter, and on some days
Speaker 1: there won't be any at all. A contrail forms because
Speaker 1: one of the components of jet engine exhaust is water.
Speaker 1: Jet fuel is made of carbon and hydrogen. When jet
Speaker 1: fuel burns with oxygen, most of the exhaust consists of
Speaker 1: CO two or carbon dioxide, and H two O or water.
Speaker 1: A big jet burns hundreds of gallons of fuel per hour,
Speaker 1: meaning it produces hundreds of gallons of water. The water
Speaker 1: is generally an invisible vapor. When you exhale, your breath
Speaker 1: contains a great deal of invisible water vapor as well.
Speaker 1: You may have noticed that on certain days in the winter,
Speaker 1: your breath will form a cloud of condensation when you exhale.
Speaker 1: In the summer, however, you don't see your breath. You
Speaker 1: see the condensation in winter because cold air can hold
Speaker 1: a lot less moisture than warm air, so in the winter,
Speaker 1: when the moisture in your breath hits the cold air,
Speaker 1: the moisture condenses into a visible cloud. The same thing
Speaker 1: happens when a jet engine exhales. If the temperature, winds,
Speaker 1: and humidity in the upper atmosphere are right, long white
Speaker 1: trails form when the moisture in the exhaust condenses. For
Speaker 1: more on this and thousands of other topics, does that
Speaker 1: how stuff works dot com and don't forget to check
Speaker 1: out the brain stuff blog on the house stuff works
Speaker 1: dot com home page. You can also follow brain stuff
Speaker 1: on Facebook or Twitter at brain stuff hs W. The
Speaker 1: house Stuff Works iPhone app has arrived. Download it today
Speaker 1: on iTunes, m