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