The Speed of Sound
Source: Air & Space/Smithsonian.
The speed of sound (otherwise known as Mach 1) varies with temperature. At sea level on a “standard day,” the temperature is 59°F, and Mach 1 is approximately 761 mph. As the altitude increases, the temperature and speed of sound both decrease until about 36,000 feet, after which the temperature remains steady until about 60,000 feet. Within that 36,000–60,000 foot range, Mach 1 is about 661 mph. Because of the variation, it is possible for an airplane flying supersonic at high altitude to be slower than a subsonic flight at sea level.
The transonic band (the “sound barrier”) extends from around Mach .8—when the first supersonic shock waves form on the wing—to Mach 1.2, when the entire wing has gone supersonic.