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Vapor trail.


A quick-acting photographer recently snapped this image of a fighter jet rocketing through a funnel-shape cloud. Vapor cones like the one pictured here sometimes appear when aircraft approach the speed of sound. This speed varies depending on the temperature of the medium the sound waves are moving through. For example, sound waves in 20[degrees]C (68[degrees]F) air travel at a whopping 1,207 kilometers (750 miles) per hour.

As aircraft accelerate to superfast speeds, the air flowing around them also speeds up. That causes a drop in air pressure and temperature. "If the air is humid enough, the temperature change will cool the air enough to cause condensation," says Peter Coen, leader of NASA's Supersonic Research Project. As water in the air changes from a gas to a liquid, a vapor cone forms.

Vapor cones taper off and end abruptly due to the presence of a shock wave. These powerful sound waves result from a sudden change in pressure, velocity, and temperature. When an aircraft finally goes supersonic, or exceeds the speed of sound, shock waves form at the plane's taft and nose. People on the ground hear the wake of these sound waves as a loud sonic boom.

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Title Annotation:PHYSICAL/SOUND
Author:Crane, Cody
Publication:Science World
Article Type:Brief article
Geographic Code:1USA
Date:Oct 5, 2009
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