The speed of an object that is travelling faster than the speed at which sound waves propagate in the medium through which it is moving. At sea-level pressure (0.1 MPa), in dry air at a temperature of 20°C, sound travels at about 1225 km/h. This is known as Mach 1, and the Mach number is the ratio of the speed of the body to that of sound under prevailing conditions. The speed of sound varies with the density of the medium; sound travels through water at room temperature at about 5184 km/h. Consequently, the Mach number represents different speeds, and in air it varies according to the temperature. An aircraft flying at Mach 1 at an altitude of 12 km in air at –56.5 °C is travelling at 1062 km/h, but to reach Mach 1 at a height of 6 km, with an air temperature of –24.6 °C, it must fly at 1138 km/h.