How a Soaring Plane Marks Its Trajectory

Question: Why do jets leave a white trail in the sky?

Answer: The cloud-like trails--called contrails, short for “condensation trails”--are produced when water vapor in the engine exhaust condenses and freezes in the cold atmosphere. The same effect is seen when an automobile is started on a cold morning or when people exhale in cold air. In those cases, however, the “clouds” do not persist because the humidity in the air is low. How long the contrails persist is a reflection of atmospheric conditions and can be used to predict the weather. A thin, short-lived contrail indicates low humidity, a sign of fair weather. A larger, longer-lasting contrail can be formed when the air is very humid and additional water vapor from the atmosphere freezes onto the ice particles emitted from the plane; such a contrail suggests a storm is coming.