Dr. Christopher S. Baird

# Can wind travel faster than the speed of sound?

Category: Earth Science      Published: January 10, 2014

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Wind travels faster than the speed of sound in supersonic wind tunnels. Public Domain Image, source: NASA.

Yes, wind can travel faster than the speed of sound. Wind is just the bulk movement of a mass of air through space and is in principle no different from a train speeding along or a comet zipping through space. The only limitation on the bulk motion of objects with mass is the universal speed limit of the speed of light in vacuum. The speed of sound is not a fundamental speed like the speed of light is. The speed of sound just describes how fast a mechanical wave travels through a material. Different materials have different speeds of sound. Wind speed and the speed of sound are completely different things. Wind speed is the speed of a chunk of air moving relative to a stationary, external observer. Wind speed is therefore frame dependent. On the other hand, the speed of sound is the speed of a vibrational wave inside a material relative to that material, and is therefore not frame dependent. If you pick your frame of reference right, you can get wind traveling at nearly the speed of light. For instance, air in your living room that is stationary with respect to you and the earth is also wind traveling at incredible speed relative to the center of our galaxy. Air on the International Space Station is traveling about 20 times the speed of sound relative to earth, and yet the astronauts in the space station can talk just fine and send sound back and forth through this air. With that said, large gusts of wind on earth's surface do not typically travel anywhere near as fast as sound.

So what would happen if wind traveled faster than the speed of sound? In isolation, not much. Wind traveling faster than the speed of sound in isolation is still just wind. But, if that supersonic wind hits a stationary object, it will create a sonic boom and knock that object forcefully. A jet flying faster than the speed of sound through stationary air is identical to wind blowing faster than the speed of sound past a stationary jet. In both cases, the same sonic boom is created. In fact, this equivalence is the operating principle behind wind tunnels. Wind going faster than the speed of sound and hitting stationary trees, houses, and rocks would create sonic booms. Note that if there really were large, sustained, weather-caused winds that were going faster than the speed of sound relative to the ground, you would have a lot more to worry about than the sound of the sonic booms. The speed of sound in air is about 750 miles per hour (340 m/s). Sustained wind speeds above 60 miles per hour are enough to uproot trees, knock down buildings, and throw cars around.