What are the white trails those planes leave on the sky and what's its purpose?
The white trails that planes leave behind are officially called contrails, short for concentration trails [1, 2]. They are formed when the water vapor in the exhaust from the plane's engines condenses into water droplets, which then freeze into ice particles composing a line-shaped cloud [1]. The impurities in the jet exhaust form some of the particles on which water droplets grow, before freezing [1]. Contrails are considered a minor component of pollution and climate change; however, they do contain exhaust, which is similar to the way automobiles emit exhaust [2].
The white trails that planes leave behind are called contrails, which is short for "condensation trails" [4]. They are not smoke from the engines, rather, they form when the water in jet engine exhaust mixes with wet cold air and condenses and freezes into ice crystals [4]. Jet engine exhaust contains a lot of water vapor, as well as carbon dioxide, oxides of sulfur and nitrogen, unburned fuel, soot and metal particles [1, 3]. The soot provides condensation sites for the water vapor, and any particles present in the air can provide additional condensation sites [1, 3]. When the air is wet and cold enough, the trails can stay around for a long time and sometimes spread out [4]. Contrails are actually a type of cirrus cloud [4].