Travelling at the Speed of Sound

Today, Felix Baumgartner broke records and the sound barrier by jumping out of a balloon from a height of a little over 39km. His aim was to break the sound barrier in freefall.

The speed of sound in air drops slightly as the temperature drops:

Anyone watching the live coverage will have seen the extremely low temperatures in around the middle of his ascent: the temperature dropped to -68°C at one point, before rising to -25°C by the time he reached floating height.

In the middle of his ascent, with those low -68°C temperatures, the speed of sound would have been down to 287 m/s (about 1033 km/h), compared with 343 m/s (1235 km/h) at 20°C.

By the time he got to the top of his ascent, with temperatures up to -25°C, the speed of sound was up to to 316m/s.

Dropping from 39km gave him 19km of low resistance freefall in which to get to the region with the lowest temperatures, at around 10-20km altitude. Assuming he was within that region when he achieved his peak speed of 373 m/s (1,342.8 km/h), he would have been travelling at 1.30 times the speed of sound in that region.

The Mach number for speed is a multiple of the speed of sound in any particular medium, so its value changes depending on the medium. This means that at -68°C in air, 373 m/s is Mach 1.30.

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