Lexi: "Hi Noelia! I will be glad to explain that to you: The white trails are so to say artificial clouds that may be caused by the emissions of the airplanes if the air is particularly cold and moist.
You see: So that an airplane can fly, it needs fuel - like cars need petrol for instance. In the case of airplanes the fuel is called kerosene. The kerosene is burnt in order to power the plane. Different substances are emitted in the process - among others steam and tiny particles of soot.
When the aircraft emits steam, so-called condensation comes into play. That is what it is called when a gaseous substance (like the steam in our case) changes into a liquid state."
Bernie: "Ah I see: Where the airplanes fly the air is very cold and often also moist. The steam cools down so quickly that it becomes drops of water - and then it freezes into ice crystals!"
Lexi: "Precisely, Bernie. The steam condenses - by the way that is why the white stripes are called condensation trails. The small pieces of ice that form stick to the particles of soot that the aircraft also emits. When a high number of these particles collect we can see them from the ground as white stripes."
Bernie: "And because the air has to be so cold for this to happen, the trails only form if the airplanes are flying very high up and not when they take off or land. That's logical!"