this post was submitted on 10 May 2025
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Vapor dispersal. (ani.social)
submitted 4 days ago by MyDarkestTimeline01@ani.social to c/memes@lemmy.world
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[–] yesman@lemmy.world 6 points 3 days ago (1 children)

I think it might be more about temperature differences than pressure differences. That is to say hot exhaust cools rapidly and any water vapor condenses. Some aircraft leave no contrails, depending on atmospheric conditions.

Here is a chart to predict contrails on a high-bypass jet engine

And here are aircraft leaving contrails without any jet engines

And some more leaving no contrails at all

[–] Lucien@mander.xyz 4 points 3 days ago* (last edited 3 days ago) (1 children)

The temperature difference is caused by the pressure difference. Airplanes have always caused pressure differentials. Jet engines just cause more pressure than wings and propellers do.

[–] Eheran@lemmy.world 5 points 3 days ago* (last edited 3 days ago)

It has next to nothing to do with pressure, let alone temperature drop due to expansion. There are 2 things:

  1. When each one quantity of cold and warm air mix, the temperature of the mixture is almost exactly the midpoint (average), as the heat capacity is almost a constant.
  2. Vapor pressure of the water is a function of temperature and scales FAR more than linear.

So now when the hot, humid (burned hydrocarbon) air of the exhaust mixes with cold air the temperature drops a bit, but the vapor pressure drops massively. When conditions are right, the vapor pressure is now below the amount of vapor pressure that is actually present -> condensation.

vapor pressure over temperature data, note how it changes more than 2 orders of magnitude over only 100 K.

Just found this from NASA.