Matthew Stuzynski, age 12, of Omaha, Neb., for his question:

WHAT IS A CONTRAIL?

That thin line of cloud forming behind a high flying aircraft is called a contrail. It is made up of tiny water droplets or ice crystals. They form when water vapor condenses in the air or freezes.

Contrails are also called condensation trails, exhaust trails or vapor trails.

You'll find that one of the ways a contrail forms is when a plane moves through the air and the pressure over the aircraft's wings drops. As the pressure drops, so does the temperature of the air, and the water vapor in it may condense. A contrail may also form from an aircraft engine that contains water vapor. A third way can produce the contrail from the engine's exhaust. The pressure and temperature of the air drops as the air rises, causing the water vapor in the exhaust to condense.