Kirk Evans, age 9, of Reno, Nev., for his question:

WHAT CAUSES A JET'S CONTRAIL?

A contrail is a thin line of cloud that forms behind aircraft at high altitudes. Contrails are made of tiny water droplets or ice crystals. Contrails are also called condensation trails or vapor trails.

A contrail may form in three ways: when a plane moves through the air, the pressure of the air over its wings drops. As the pressure drops, so does the temperature of the air and the water vapor in it may condense.

A second way: the exhaust from an aircraft engine contains water vapor. This vapor may condense when it reaches the cold air around the plane.

Number three: air rises if heated by an aircraft engine or its exhaust. The pressure and temperature of the air drop as the air rises, and may cause the water vapor in it to condense.

Ice crystals from a contrail may cause rain or snow to fall from certain clouds. The crystals act like the chemicals men use when "seeding" clouds to make rain.