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Originally Posted by lucrum
Hot humid air from the engines...I might be wrong here, but as far as I know a jet engine does not add humidity to air unless it is capable of providing an internal source of hydrogen and oxygen to make new water molecules that add to the already existing amount in the surrounding air. Combustion in a jet uses oxygen to fuel the flame, therefore if anything the humidity could possibly be lower on the exhaust side of a jet.
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Yes, you've got it wrong. A jet engine DOES add humidity to the air.
Jet fuel, like most petroleum derivatives is a HYDROCARBON. It is made up primarily of hydrogen and carbon, hence the name. Hydrogen burns by combining with oxygen from the air to produce water. Carbon burns by combining with oxygen from the air to produce Carbon Dioxide (or Carbon Monoxide if it burns incompletely).
Science experiment: wax is a hydrocarbon. Hold a cold mirror or piece of glass over a candle and notice that water condenses on it.
Don't they teach this stuff in science classes any more?
This NOVA article on contrails provides more information on the conditions under which contrails persist and even grow and spread. Scientists have long been concerned about the possible effects of contrails on climate. The shutdown of all air traffic for three days following 9/11 2001 provided a unique opportunity to study individual contrails and to measure conditions when there were no contrails:
Dimming the Sun: Are vapor trails from aircraft influencing the climate, and if so, should we worry?
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If conditions are right, newly formed contrails will begin feeding off surrounding water vapor. Like vaporous cancers, they start growing and spreading. In time, they can expand horizontally to such an extent that they become indistinguishable from cirrus clouds, those thin, diaphanous sheets often seen way up high. These artificial cirrus clouds can last for many hours, and the amount of sky they end up covering can be astonishing: one study showed that contrails from just six aircraft expanded to shroud some 7,700 square miles.
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