Why do some airplanes leave a visible contrail and others don’t?
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Interesting note; it’s not just jets that make contrails. It’s simply that jets are generally the only planes that can fly high enough to make contrails. Back in World War 2 it was common for bombers to be seen making contrails: [https://www.google.com/search?q=ww2+bombers+contrails&safe=off&client=firefox-b-1-d&sxsrf=ALeKk00DfHVemgMi0MMYbzwrxk8ObQnZPg:1606255860856&tbm=isch&source=iu&ictx=1&fir=khTCWyNVDcV_3M%252Cj-5mWhZ5KdfXGM%252C_&vet=1&usg=AI4_-kQGJ1Z7o-e-N_vQTpJnS5u-qn0EHg&sa=X&ved=2ahUKEwjTvISwmZztAhWBtVkKHRtpCOUQ9QF6BAgFEDM&biw=1920&bih=966#imgrc=khTCWyNVDcV_3M](https://www.google.com/search?q=ww2+bombers+contrails&safe=off&client=firefox-b-1-d&sxsrf=ALeKk00DfHVemgMi0MMYbzwrxk8ObQnZPg:1606255860856&tbm=isch&source=iu&ictx=1&fir=khTCWyNVDcV_3M%252Cj-5mWhZ5KdfXGM%252C_&vet=1&usg=AI4_-kQGJ1Z7o-e-N_vQTpJnS5u-qn0EHg&sa=X&ved=2ahUKEwjTvISwmZztAhWBtVkKHRtpCOUQ9QF6BAgFEDM&biw=1920&bih=966#imgrc=khTCWyNVDcV_3M)
Why do some dissipate quickly and others stay for a long time?
Assuming you live somewhere where it gets cold, you may have noticed that somethimes your breath becomes visible when you’re outside and it’s cold. You’re still the same person (approximately 😉 ) as you were in summer, when you couldn’t see this.
This is due to the fact that air can hold some amount of water, but how much that is depends on its temperature, the colder it is, the less water it can hold, before the water turns (condenses) into visible little droplets (i.e. your breath, contrails) again. When air is at a set temperature but holds little water, you need do add quite a bit before it condenses and you can see it. Conversely, if there’s a lot already, just a little bit is enough to observe condensation. So moist and or cold air: contrails very likely; dry / warm air: contrails unlikely.
So for an airliner at a cruise altitude of over 30 000ft where temperatures might be -60°C, you’ll quite often see contrails, if you’re looking at a small propellor driven aircraft at a few thousend feet it will only be slightly colder than on the ground, so no contrails. And in those two cases there’s also a significant difference in fuel consumption, which has a direct influence on how much water there is in the exhaust, and the fact that a propellor creates turbulence which dilutes the exhaust gasses somewhat, too.
Contrail formation generally depends on the altitude, air temperature, air pressure, and ambient humidity. They are mainly seen in aircraft flying above, say, 8000 m in altitude.
If the air is cold enough it will rapidly cool the hot engine exhaust, causing it to condense into fog. This is the same effect as exhaust from cars condensing in freezing weather, or fog produced from your hot exhaled breath. In most conditions this fog doesn’t persist for long, as the tiny cloud droplets evaporate again.
If the air temperature is significantly below zero, the cloud droplets will then freeze into microscopic ice crystals. Not only that but they will slowly grow in size by picking up additional water vapor from the surrounding air if conditions are right. The latter is what causes long persistent contrails. The air is so cold at that altitude that the airplane exhaust becomes a locallized snowstorm of very fine ice dust. Eventually the ice crystals sink down to a layer of warmer air where they evaporate again.
Contrail formation has many factors. The height of the plane, the speed of the plane, the temperature of the air, the humidity of the air, and many other things go into it.
Change enough of those things, and you get different results.
It does not depend so much on the airplane as the humidity, pressure and temperature of the air in flies in. Contrails is an atmospheric effect that is caused by the increased humidity from the engine exhaust raising the humidity in the air high enough to form clouds. There is a few other effects such as sot creating nucleation points for the ice crystals and turbulence creating low pressure spots for crystals to form but these are of minor significance. Contrails will be formed whenever an airplane moves through air that is close to forming clouds already. The humidity from the exhaust is just the final straw that makes the clouds form behind the airplane. If you see two airplanes in the same sky where one forms a contrail and the other do not then it is likely because they are passing through different layers of the sky with different conditions.
I never saw contrails as a kid in the 80s is this because there were less planes flying round?
Really short answer with another question: why can you see the exhaust from some cars on some days but not on others?
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