why do planes leave trails

Planes leave trails known as contrails, which are short for "condensation trails." These trails form when water vapor produced by the combustion of fuel in the aircraft engines mixes with the cold, low-pressure air at high altitudes (usually above 8,000 meters or about 26,000 feet). The water vapor rapidly condenses into tiny water droplets and quickly freezes into ice crystals, creating visible, white line-shaped clouds behind the aircraft. The cold temperatures at cruising altitude (around -55°C) enable this process. Persistence and length of the contrails depend on atmospheric conditions such as humidity; if the air is humid, contrails can persist and spread for minutes to hours, while in dry air they dissipate quickly. Additionally, contrails form because the exhaust particles (like soot and sulfur compounds) act as nuclei for condensation. Also, pressure changes caused by the aircraft's wings and engine turbofans can contribute to contrail formation. These are the visible white trails commonly seen trailing behind planes in the sky and are composed mainly of ice crystals formed from aircraft exhaust and atmospheric moisture.