What are those white lines left by airplanes in the sky, and where do they come from?

Why do they appear one day… and not the next?

The question often comes up: if planes fly every day, why don’t we always see these contrails? The answer lies in atmospheric conditions. When the air at altitude is too dry, the crystals evaporate immediately: the trail disappears in a few seconds.

Conversely, when the air is humid, these artificial clouds persist longer, sometimes for several minutes, sometimes even for a large part of the afternoon. Much like a meringue that holds its shape because the right balance has been found, the atmosphere must be in a very specific state for these contrails to last.

Snow trails, a valuable clue for anticipating the weather

What many people don’t know is that these lines in the sky are real indicators for specialists. Their duration, shape, and dispersion can help predict weather patterns.

When contrails persist for a long time, it can reflect a more humid atmosphere at altitude, often a harbinger of a change in the weather, such as the arrival of a disturbance or a veil of cloud. It’s a bit like when you feel the air becoming heavy before a downpour, but this time at an altitude of over 10,000 meters!