Why do some airplanes leave a visible contrail and others don’t?
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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.
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