[RidaChat (OP)]

Why planes leave trails in the sky ?

[Bored chemist]

Why planes leave trails in the sky ?

Largely, for the same reason that cars and lorries do. They leave a trail of exhaust fumes.
There's also a second effect- they plane causes condensation of water vapour from the air.
https://en.wikipedia.org/wiki/Contrail


Incidentally, since this has nothing to do with the appearance of propellers, you should have started a separate thread for it.

[Colin2B]

Why planes leave trails in the sky ?

Incidentally, since this has nothing to do with the appearance of propellers, you should have started a separate thread for it.

Quite right, I’ve split it off

[evan_au]

Exhaust gas from a car contains a lot of water vapor, but it is so hot that it stays a vapor until it disperses.

At the altitude that jets fly, the air is so incredibly cold that the water vapor in the exhaust cools close behind the plane, forming a trail of ice crystals in the air.

See: https://en.wikipedia.org/wiki/Contrail

[RD]

... this has nothing to do with the appearance of propellers, you should have started a separate thread for it.

There can be condensation trails at the tips of propellers & tips of wings ...





https://en.wikipedia.org/wiki/Wingtip_vortices

[Bored chemist]

"Why do aeroplanes leave trails in the sky?"
Where else?

[evan_au]

Trails behind airplanes only occur under particular conditions of temperature and humidity. This is often restricted to a narrow band of altitudes.

There are two different mechanisms at work in these examples:
- The trails from the tips of wings and propellers occur due to turbulence, causing moisture already in the air to condense into droplets big enough to disperse visible light. These trails start right at the body of the plane. But these vortices usually dissipate  enough in one or two plane-lengths to become invisible again*.
- The trails coming from the engines is due to extra moisture added to the air from the burning fuel. These trails start somewhat after the engines, as the exhaust needs to cool down before the water vapor can condense and/or freeze. Under the right conditions, these trails can persist for many minutes after the plane has passed.

One of the important tasks of the weather bureau is to advise military aviators what altitudes to avoid. The last thing you want on a stealthy military mission is a big arrow across half the sky pointing at exactly where you are!

*Just because these vortices are invisible does not mean that they are harmless. You do not want to pilot a light aircraft anywhere near the flight path of an A380!

"Why do aeroplanes leave trails in the sky?"
Where else?

They often leave rather dramatic skid marks on the threshold of the runway...

[alancalverd]

Quote from: Bored Chemist"Why do aeroplanes leave trails in the sky?"Where else?They often leave rather dramatic skid marks on the threshold of the runway...
                           
                           

Which is just as well. I must have flown into Leeds-Bradford a dozen times, but somehow they keep moving it between trips and if you make a visual approach (below 2000 ft in the controlled area) it disappears behind the city or under a hill - twice last week! But the runway is immediately identifiable by the black skidmark on a white surface. Who needs GPS?