vapor trails to you...

I’m confused. What are those trails left behind jets. I’ve been told two stories, both by (supposedly) reputable pilots. One claims that the trails are just water vapor and are produced as a function of moisture content in the air and vehicle speed. Another “reliable” source claims that the jets are actually venting gasses that (1) affect engine performance and (2) would be harmful if released near the surface. I watched as the shuttle left an incredible vapor trail across the Texas sky last night and my first reaction was that the first explanation must be right since the shuttle doesn’t use her engines to land… but then I thought… well, maybe she’s venting something else…

Just curious…

There are several things at work here. First, jet engines are like any other engine which burn fossil fuels. The fuel combines with oxygen in the air to produce carbon dioxide and water (assuming totally clean combustion; in actuality they produce an assortment of hydrocarbons and other by-products as well).

The water in the exhaust is in the form of water vapor which condenses in the colder upper atmosphere to produce the vapor trails you see.

Second, the other by-products include small particles. (Think of them as soot.) These small particles provide condensation sites for water vapor in the air. This also can result in a vapor trail.

Finally, airflow over and around the aircraft creates areas of high and low pressure. Lower pressure causes the water vapor in the air to condense. This is probably what caused the trail behind the shuttle.

There are probably other factors at work as well but I don’t think the trails are caused by the aircraft dumping anything.

“You can’t run away forever; but there’s nothing wrong with getting a good head start.” — Jim Steinman

Dennis Matheson —
Hike, Dive, Ski, Climb —

Just a side note:

Sometime in the last week or two, on the front page of the Globe and Mail (Toronto paper), there was a photo, taken with a high-speed camera, of a Hornet fighter plane from the carrier Constellation just passing the speed of sound. The vapor trail was just starting, as a disk around the tail of the plane. Very impressive!

I assume it went out on the newswires, but I couldn’t find it on the Web.

Bob the Random Expert
“If we don’t have the answer, we’ll make one up.”

I saw the same picture in our local paper in Connecticut.
They also stated that a skilled pilot could move the shockwave forward and backward on the aircraft by using the throttles.



“Moderate strength is shown in violence, supreme strength is shown in levity.”~~G.K.Chesterton 1908

When I started flying in jet aircraft, I tried to see the contrails from inside. I couldn’t. Then I observed, from the ground with binoculars, they form a plane length or two behind. It’s the same as in cold weather, we see the exhaust from cars, often a few inches beyound the pipe. What was seen behind the shuttle was like we see behind a metior. Plasma.


They are usually just the odd atmospheric thing. However pilots will, under emergency conditions, jettison fuel. This looks markedly like a contrail. Certain jet engines do vent gasses (gas producing turbines) but these are mostly compressed air and do not create or contribute to contrails. This type of engine is typicaly used to power turbo prop craft. Any aeronautical engineer may feel free to correct me, but engines that use exaust thrust for motive power do not employ a bleed band. (A dohicky that vents engine gas) So your friend is pulling your leg.