I feel like when I look up, 9/10 time there’s going to be a trail behind the plane, but if I go on a flight I never see it happen. Is it a specialty plane or something?
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It’s a condensation trail, or contrail.
They are created by jet engines compressing and heating the air traveling through them. As the exhaust expands and cools any moisture in the exhaust quickly condenses and forms a cloud.
Whether or not a jet engine produces a contrail depends on the amount of humidity in the air. It may also depend on the type of jet engine and the fuel its burning. There are many ways in which military jets reduce or even eliminate contrails.
When you travel on a jet, you may not see the contrail because it is forming well behind the airplane. You also just don’t have a very good angle from the passenger compartment to see much of anything.
They are [contrails](https://en.m.wikipedia.org/wiki/Contrail) or mostly ice crystals formed around impurities and water vapor (from jet exhaust).
[Why Don’t All Planes Leave Smoke Trails?](https://www.mentalfloss.com/article/23644/why-dont-all-planes-leave-smoke-trails). Edit: temperature and humidity mostly.
[Chemtrail conspiracy theory](https://en.m.wikipedia.org/wiki/Chemtrail_conspiracy_theory): or maybe someone is spraying chemical or biological agents from planes…
The engines are super hot and the air at cruising altitudes is really cold so when the air passes through the jet engine it is heated and then instantly condenses when re-exposed to the high altitude air into a cloud. So you are seeing a cloud trail behind the plane
The trails are made of water condensation from the engine exhaust. You don’t see it from the plane because the engines throw out several hundred degree gases so the trails only cool down enough for condensation to appear a bit [behind the vehicle.](https://upload.wikimedia.org/wikipedia/commons/thumb/b/b0/A340-313X.jpg/1920px-A340-313X.jpg)
Engine exhaust is mainly co2+water
Water + very cold environment – > water droplets which freeze – > cloud
Yea, so due to the way a wing’s shape enables the plane to fly. It’s splits the air and the trail you see is a disturbance in the air. If you look further behind the plane you can often see the trail disappearing as the air molecules all settle back into place. It can be seen from the plane but it must be under the right conditions. There are conspiracy theories that you might hear about regarding “chem-trails” these are false.
Edit: it’s has been made clear I am completely wrong.
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