Contrails are formed by water vapor and ice crystals from airplane engine exhaust in cold, humid air.

When a plane flies high, its hot engine exhaust meets cold air. The water vapor in the exhaust freezes into tiny ice crystals, making white trails called contrails. How long these trails last depends on the humidity. Sometimes they vanish quickly; other times they spread out and hang around for a while, which can look unusual but is perfectly normal weather stuff.