Why do some white trails behind planes fade quickly while others last?
The short answer
A white trail behind a plane is called a contrail. Whether it fades quickly or lasts depends on the temperature and moisture in the air at the plane’s altitude.
How does a contrail form?
Jet exhaust contains water vapor. When the hot exhaust mixes with very cold air high above us, the vapor changes into water droplets around tiny particles. The droplets quickly freeze into ice crystals. The crystals scatter sunlight, so the white trail is a cloud of ice, not smoke.
Why do some trails disappear while others remain?
When the air is dry, ice crystals change directly into water vapor without first becoming liquid. This change is called sublimation. As the crystals disappear, so does the contrail.
When the air at flight altitude is moist enough for ice crystals to grow, water vapor from the surrounding air adds to them. The trail may then remain visible for hours or spread. Meteorologists call this state ice supersaturation.
The temperature and moisture in the air on the ground can differ from conditions at flight altitude. Ground weather alone cannot tell you how long a contrail will last.
Why do contrails spread out?
Winds high in the atmosphere carry long-lasting contrails. Winds that differ in speed or direction at different heights can bend or widen a trail. It may also appear to start or stop when a plane enters or leaves a moist area of air.
Can a white trail form near the wings?
A short white trail can also form near an airplane’s wingtips. In very moist air, lower pressure around a wing cools the air. A swirling flow behind the wing can help water vapor form tiny droplets or ice crystals. This is a different process from an exhaust contrail and happens under limited conditions.