NEW JERSEY - Is it a drone, a plane or a helicopter in the sky? Experts who study unmanned aircraft systems, better known as drones, say it can be tough to tell from miles away. But there are clues.

Legends of the Sky: The Liberty Plane is available to stream on pbs.org and the free PBS App, available on iPhone, Apple TV, Android TV, Android smartphones, Amazon Fire TV, Amazon Fire Tablet ...

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.