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Name: Charlie
Status: other
Grade: 9-12
Location: CA
Country: N/A
Date: 5/31/2005
Question:
What causes the jet stream that is caused by aircraft? Is it dependant on the altitude of the aircraft? Do passenger aircraft have enough speed to cause a vapor trail?

Replies:
Charlie,

Most jet contrails are produced from passenger aircraft. On a clear summer day it's not unusual to see the sky criss-crossed with jet contrails.

The temperature of the air is more important than the altitude or the speed of the jet. Jet fuel (like most fuels) contains water, which is vaporized by the heat of the jet engine. This is expelled into the air behind the airplane and mixes with the very cold air. As the jet contrail cools to the dew point or frost point, the water vapor turns into water droplets, or more often ice crystals, forming the visible contrail.

Something similar happens with the exhaust from the tailpipe of a car during the cold temperatures of winter; you see a visible plume of water vapor as the exhaust from the car cools. You will also see this happening in the exhaust from a power plant stack. The principle is the same.

David R. Cook
Atmospheric Physics and Chemistry Section
Environmental Research Division
Argonne National Laboratory

Dear Charlie-

Contrails are "clouds" produced by the exhaust of aircraft. At high enough altitudes where the temperatures are cold enough, the combustion byproducts contain enough moisture and condensation nucleii to cause the moisture to condense and form clouds along the track of the aircraft. These contrails usually spread out and dissipate as the cloud mixes with the ambient air. But sometimes, if conditions are right, the clouds persist, and can even expand, sometimes spreading nearly over the whole sky.

Here is a link that has a good discussion of contrails and some photos.

http://cimss.ssec.wisc.edu/wxwise/contrail.html

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO

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