Contrails are human-made clouds. They are streams of condensation left behind from airplanes and rockets. Exhaust leaves water vapor that freezes into tiny crystals in the cold troposphere.

The water vapor from the hot plane engine turns to ice mid-air which we see as white lines in the sky called contrails. This is the same phenomenon we see when we breathe hot air on a cold day.

Contrails are an instant visual reminder of air travel, and a source of much controversy in recent decades. Put aside the overblown conspiracies, though, and there are some genuine scientific ...