Why did the plane pull the white line?

That's not a white line. In fact, it is because when the plane flies at high altitude, a lot of exhaust gas will be ejected from the back of the tail, which will condense with water vapor in the air into many small water droplets, which looks like dragging a white belt or line. This phenomenon usually only occurs on sunny days.

According to the formation reasons, aircraft wake can be divided into three types: exhaust wake, convection wake and aerodynamic wake. Exhaust wake can be divided into exhaust condensation wake and exhaust evaporation wake, and the common aircraft wake is exhaust condensation wake. After the plane passed by, the engine spewed out high-temperature gas containing a lot of water vapor. If the ambient temperature is very low (below -40℃), these high-temperature and high-humidity gases quickly mix with the ambient air, the temperature drops rapidly, and the humidity increases sharply, thus condensing to produce aircraft wake, which is completely different from the effect of adding smoke screens in flight shows.

The formation of aircraft wake is not only related to the temperature of the surrounding atmosphere, but also depends on the temperature of flight altitude. Experience shows that when the temperature is between -40℃ and -60℃, the humidity is above 40%, or the humidity near the tropopause is above 60%, the aircraft wake may appear. The exhaust wake mostly occurs at the height of 1 1 ~ 13 km above the ground.