Ask A Scientist Weather Archive

name         Charlie
status       other
grade        9-12
location     CA

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?
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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
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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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