Well Mach is a term that defines the speed of sound.
This term is used mostly in fighter aircraft to express the speed of the aircraft.
The mach depends on some variables the most important ones
are : altitude and temperature.
When the altitude changes the air-pressure changes with it so that the speed of the sound changes too.
When the temperature changes it effects the density of the air and by that effects the Mach number.
The speed of sound on S/L(sea-level) is approximately :
1,214 km/h

to go back click the back button on your browser