Barton Rifkind, age 15, of Twin Falls, Ida., for his question:

HOW IS A JET'S CONTRAIL FORMED?

A contrail is a thin line of cloud that forms behind jet aircraft at high altitudes. Contrails consist of tiny water droplets or ice crystals. They form when water vapor in the air freezes or condenses and becomes liquid.

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

A contrail may form in one of three ways.

(1) When a plane moves through the air, the pressure of the air over the wings and around the engines drops. As the pressure drops, so does the temperature of the air and the water vapor in it may condense.

(2) The exhaust from an aircraft engine contains water vapor. This vapor may condense when it reaches the cold air around the plane.

(3) Air rises if heated by an aircraft engine or its exhaust. The pressure and temperature of the air drop as the air rises, causing the water vapor in it to condense.