Why do airplanes leave long white lines when flying through the sky?

Sometimes when you look up at the sky, you'll see a long white line trailing behind an airplane, looking like someone drew a stroke in the sky with chalk. This white line is called a 'contrail,' and it's not white smoke emitted by the plane. When airplanes fly at high altitudes, the exhaust from their engines contains water vapor. When this water vapor meets the extremely low temperatures at high altitudes, it can condense into tiny water droplets, which then form ice crystals, creating the white lines we see. Interestingly, not every plane leaves a visible contrail. If the air at high altitudes is dry, the ice crystals formed will dissipate quickly; if the air is humid, the white line might persist for a long time, or even spread out into a thin layer of clouds. So, the reason you sometimes see planes leaving long white lines and sometimes not at all is actually related to the temperature and humidity of the air at high altitudes at that time. Next time you see a plane streak across the sky, try to pay attention to how quickly the white line disappears. A white line in the sky might seem like just a trace of flight, but it actually hides little secrets about the atmospheric environment.