Why do planes leave the white mark in the sky?

2000/05/17 Elhuyar Zientzia

• Garraioak
• Aeronautika
• Fisika

The white footprint left in the sky by the planes has a simple explanation: this white footprint is only condensed water.

In the engines of the plane, when the fuel is burned, water is generated in a state of steam that the plane expels together with the hot air. This steam ejected does not always condense, so the planes do not always leave the mark white. In order for the planes to leave a mark, or in other words, to condense the steam expelled, the atmospheric air must be cold and humid.

When the atmospheric air is cold and humid, the steam released by the engines condenses and we see the white trail in the sky. These temperatures are very low and with a high degree of humidity, usually at high altitudes. That's why only very tall planes have a white tail.

Sometimes, the steam is not only condensed, it also freezes and although the plane has long disappeared, the footprint takes hours to disappear completely. To explain it in a simple and understandable way, let's say that the phenomenon of the white footprint left by the planes is the same that occurs when we breathe in the cold winter.