Well, it’s not jet exhaust you’re seeing, though. It’s water vapor in air that’s been compressed in a jet engine and then quickly decompressed out of the back, which causes the air to cool thus condensing the water vapor into droplets similar to those in a cloud.
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
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.
It has next to nothing to do with pressure, let alone temperature drop due to expansion. There are 2 things:
- 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.
- 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.
And those clouds, being too thin to reflect sunlight but able to trap heat, form a kind of blanket around earth, greatly contributing to global warming.
If I understand it correctly, you could magic away the exhaust and have perfectly clean contrails, and they would still warm the planet.
Close, the visible parts do reflect sunlight, but what gets through still gets trapped by the greenhouse gasses. You can see the effect in the aftermath of the mass grounding of aircraft on 9/11. There were crystal clear skies and a relatively large jump in temperature compared to just before and after the grounding.
Incorrect. The condensation you are seeing in the air is a product of combustion. If no water was added to the air, then compressing it and decompressing it would not create a cloud or vapor trail.
Edit: Fine. It’s mostly incorrect.
https://en.wikipedia.org/wiki/Contrail
Yes, the lower pressure can create a very temporary contrail, and in rare situations where the engine exhaust also cools down below freezing before it reaches ambient pressure, ice crystals can form and create a longer more visible contrails. Realistically, what you are seeing in a contrail is water vapor from combustion. Seriously, H2O and water are the largest products of combustion, and it’s like 99% of what you are seeing behind aircraft.
The longetivity of a contrail is determined by the atmospheric conditions. In sufficiently dry air no contrail is formed as the water introduced by the combustion isn’t enough to raise the frost point to the ambient temperature. More humidity means that the contrail may form and even persist for a while without sublimating away (Cirrus homogenitus). If the air is even more humid, the contrail may even start to grow and change shape (turning into Cirrus homomutatus). This last case is what I suspect gets the foil hats most riled up about “chemtrails”.
https://cloudatlas.wmo.int/en/explanatory-remarks-and-special-clouds-cirrus.html
Just because the contrails are science
I understand that part, doesn’t stop them from containing the plane exhaust which is directly contributing to global warming
Well, it’s not jet exhaust you’re seeing, though.
I did acknowledge that, by the way. Jet fumes certainly contribute to global warming. I wasn’t intending to imply they don’t. Simply that it’s not jet fumes you’re seeing in contrails.
I will convolute this conversation further by stating that contrails (like all other clouds consisting of ice crystals) warm up the planet by letting shortwave radiation from the Sun through while being more reflective in longer infrared wavelenghts, thus trapping outgoing longwave radiation. Contrails themselves are also warming the planet up. It’s a small contribution in the grand scheme, but far from a trivial one.
