Airplane trails, which are also called contrails, are simply artificial clouds that originate from the exhaust of an airplanes engines. Just like other clouds, airplane trails are composed of water, H₂O, either in the form of water droplets (condensation) or ice crystals. The water originates from the combustion (i.e., burning) of airplane fuel (kerosene or gasoline), which also produces carbon dioxide (CO₂). The same thing happens when you drive your car on a cold day. The cloud like mist or water droplets coming out of a vehicles exhaust pipe on a cold day is just the water produced from burning fuel in the engine. When it is cold, this water condenses from vapor (a gas) into a liquid in the form of clouds or droplets. That is why you can see car exhaust on cold days. On warm days, the water remains in the vapor (gaseous) state and is invisible, so you cannot see exhaust on hotter days.

Airplane contrails remain in the sky for the same reason that clouds stay in the sky. Clouds consist of water in the form of tiny liquid droplets and can either evaporate into an invisible gas (water vapor), which would cause the cloud to disappear, or can remain as droplets, which causes the cloud to stay there. Whether the water droplets that compose a cloud evaporate (and the cloud disappears) and how long the cloud remains before evaporating depends upon the relative humidity of the air. Relative humidity is a measure of how much water vapor is in the air compared to how much water vapor the air can hold. It is calculated by dividing the amount of water vapor in the air by the maximum amount of water vapor the air can hold.

If the relative humidity is high, near 100%, this means that the air is saturated with water vapor. High relative humidity prevents clouds and contrails from evaporating because the air cannot hold any more water vapor and so the water stays in liquid form as a cloud. If the relative humidity is low, near 0%, this means that the air is very dry and has very little water vapor in it. Low relative humidity causes clouds to evaporate very quickly and disappear because the dry air can hold a lot more water vapor, so the liquid droplets in the cloud turn into water vapor and become invisible.

If you look at an airplane contrail in the sky, you can tell how humid the air is by how quickly the contrail evaporates. If it evaporates very slowly or not at all, and stays there for a long time, then the air has a high relative humidity and cannot hold any more water vapor. If it evaporates rapidly and quickly disappears, then the air has a low relative humidity and can hold lots of water vapor. But these conclusions only apply to the high altitude where the airplane contrails formed. The humidity on the ground where you are may be very different. Part of the reason for the difference between high and low altitudes is because relative humidity depends upon temperature. Warmer air can hold more water vapor than colder air. Therefore, if cold air and warm air have the same amount of water vapor in them, the cold air will have a higher relative humidity than the warm air because it can hold less water vapor. Air at high altitudes is colder than air on the ground, so it tends to have a higher relative humidity because it can hold less water vapor. One of the reasons why airplane contrails stay there is because the air is very cold and often has a high relative humidity where the contrails are formed.

The exhaust from a jet engine is at very high temperature and it contains numerous constituents including H₂O. When these hot gases are exhausted they mix with the thin COLD air and ice crystals form. The ice crystals are very tiny and take minutes to hours to fall out and dissipate. That is what you see a condensation plume.

Airplane wings serve as surfaces for condensation of clouds in air that is humid enough to make clouds, but not so humid that it happens spontaneously. Water droplets and ice crystals normally need condensation nuclei to form, but once they've started forming, they nucleate themselves, and keep growing as long as humidity favors it.