Why do jets leave smoke trails?

Question

Why can you only see them some of the time, and some are cut off? Why can't you see them when you're on the plane?

Answer 1

It's not smoke, it's actually a long, thin cloud called a contrail.  Jet engines exhaust mostly carbon dioxide and water vapor.  When the water vapor hits the cold air in the upper atmosphere, it very quickly condenses into water droplets and can even freeze into ice crystals.  This means you can only see them when a jet is high in the atmosphere, and the reason you can't see them on the plane is it takes a few seconds for the exhaust to cool, therefore the contrail doesn't start until a few dozen feet behind the plane.

Answer 2

Smoke trails are actually water condensing out of the exhaust and either forming clouds or ice crystals, its not smoke.  They are called "contrails".

They need time to cool and condense/freeze so you see them some way away from the plane

You can also see something like smoke on wingtips of planes when they are low but this is because the plane is "condensing" the air and making very low pressure pockets in places, making...clouds

Answer 3

It turns out that one of the products of burning fuel is water.  It's real hot when it comes out of the engine.  So you don't see it until it's farther back from the jet.  From the ground, this looks much as if it comes right out of the engine.  But binoculars will show the gap.

If the local atmosphere isn't the right temperature, the water vapor will not condense and show the clouds.