Why do some airplane contrails disappear quickly while others linger?
The short answer
Whether a contrail fades quickly or stays for a long time depends largely on the air at the plane’s altitude. In dry air, the tiny ice crystals in the trail turn back into water vapor. In moist air, they can grow, so the cloud remains.
Water vapor in a jet engine’s exhaust mixes with very cold air high above us. Tiny water droplets form and quickly freeze into ice crystals. That is how a familiar airplane contrail begins.
In 30 seconds
A contrail is a narrow cloud left behind an aircraft. The white part we see is made mostly of tiny ice crystals, not liquid water.
If the air around the trail is moist enough for ice to grow, the crystals take in water vapor and the contrail can last. In drier air, the crystals turn back into water vapor, so the trail fades soon after it forms.
How does a contrail form?
Burning jet fuel produces exhaust that contains water vapor. The air around a plane at high altitude is very cold. As the exhaust mixes with that air and cools, some water vapor condenses into tiny droplets. Those droplets quickly freeze into ice crystals.
The ice crystals scatter light, making the trail look white. So the visible line is mainly a cloud of ice crystals, rather than exhaust smoke simply hanging in the air.
Why do some trails disappear while others remain?
After a contrail forms, its ice crystals keep interacting with the surrounding air. In dry air, the ice changes directly into water vapor. This change from ice to water vapor without first becoming liquid is called sublimation. As the crystals shrink, the contrail becomes harder to see.
If the air is moist enough to support ice, the crystals take in water vapor from the surrounding air and grow. The contrail can then remain visible for a long time, sometimes for hours.
The humidity on the ground is different from the humidity at cruising altitude. A plane can leave a long-lasting trail above a dry, sunny day if the air much higher up is moist.
Why do contrails spread out?
Long-lasting contrails are carried by winds high in the atmosphere. Differences in wind speed and direction, along with turbulence left by the aircraft, can bend a thin line and spread it into a wider cloud. It may eventually look like a naturally formed cirrus cloud.
Moisture also changes from one layer of air to another. A contrail may appear in broken sections, or its appearance may change when a plane moves to a different altitude.
Can a white trail form near the wings?
Most contrails form from engine exhaust, but short white trails can sometimes appear near a plane’s wings. Air pressure can drop around a wing, cooling the air. If that air is already very moist, droplets or ice crystals may form. These trails are usually less visible and form under more limited conditions than exhaust contrails.
A common misconception
The water in a long-lasting contrail does not all come from the engine. Exhaust water vapor helps start the cloud, but ice in a persistent contrail also grows by taking in water vapor that was already in the air along the plane’s path.