How do aeroplane's produce jet streams

Allen ives recently asked about how airplanes create jet streams. Click here to see sabrinah's answer or read The AnswerBank's extended answer below:

A. They're called condensation trails or contrails. Their appearance ranges from lines stretching across the entire sky, to much shorter trails or no trail at all.

Q. What causes contrails

A. The formation of contrails is dependent upon a set of factors in the upper atmosphere: temperature, winds and humidity.

Aeroplane fuel is made of carbon and hydrogen. When it burns with oxygen, most of the exhaust emission is carbon dioxide and water. The water is generally an invisible vapour. However, when airplane exhaust is emitted, the resulting air turbulence mixes it with the low temperature and low vapour pressure conditions of the upper atmosphere and the moisture condenses into a visible cloud, which we see. This is because the colder air can't hold as much moisture as the warmer air nearer the Earth's surface.

Q. Are contrails the same as 'seeing' your breath in the winter

A. Yes, basically the warm air exhaled from your lungs, which contains invisible water vapour, hits the cold air and the moisture condenses into a cloud. This doesn't happen on warmer days because wamer air can hold the moisture in its original form, as invisible vapour.

To see another recent question and answer on condensation, click here.

Q. Some day's contrails hang around for ages, other days they disappear quite quickly, why

A. The length of time that a contrail remains intact is determined by the humidity and winds of the upper atmosphere. If the atmosphere is near saturation the contrail will last longer than in a drier atmosphere as the contrail mixes with the atmosphere and dissipates.

Q. Do contrails have an environmental impact

A. Yes, because they result in increased cloud cover. In certain areas of heavy air traffic it's been estimated that cloud cover increases by around 20%. An increase in cloud cover is linked to a reduction in heat lost from the Earth's surface and adding to global arming.

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By Lisa Cardy