Mach number

The Mach number is the ratio speed of an object to the speed of sound. It is a unitless number that is used to measure the speed of an object as it travels through some medium. It is typically used to describe the speed of fast aircraft. Since the speed of sound increases as the temperature increases, the actual speed of an aircraft traveling at Mach 1 will vary depending on the altitude and atmospheric conditions. When an aircraft exceeds Mach 1 a large pressure difference is created just in front of the aircraft. This pressure difference, called a shock wave, spreads backward and outward from the aircraft in a cone shape. It is this shock wave that causes the sonic boom heard as fast moving aircraft travels over head.