By definition, Mach 1 is exactly equal to the speed of sound in a given medium at a specific temperature. Therefore, traveling at Mach 1 is traveling at the speed of sound, while supersonic speeds are technically defined as any speed greater than Mach 1. The region surrounding Mach 1—typically from Mach 0.8 to Mach 1.2—is known as the transonic region, where some air flowing over the wings may be supersonic while the aircraft itself is still subsonic. Once an object exceeds Mach 1.0, it produces a "sonic boom" caused by the accumulation of sound waves that can no longer move out of the way of the object, creating a powerful shockwave. The actual speed of Mach 1 is not a fixed number; it varies with the temperature and the medium. At sea level in standard dry air at 20°C, Mach 1 is approximately 767 mph (1,235 km/h). As an aircraft climbs into the colder air of the upper atmosphere, the speed of sound decreases, meaning an aircraft can reach "Mach 1" at a lower ground speed than it could at sea level.