What is the Mach Number a ratio of?

The Mach number is defined as the ratio of true airspeed to the speed of sound in the surrounding medium. This dimensionless quantity helps assess how fast an aircraft is moving relative to the speed of sound, which varies with atmospheric conditions such as temperature and pressure. When the Mach number is less than one, it indicates subsonic speeds, while a Mach number greater than one signifies supersonic speeds. This relationship is crucial for understanding aerodynamic behaviors and performance characteristics at different speeds.

Other options do not accurately represent the concept of Mach number. The ratio of ground speed to airspeed (mentioned in one of the choices) does not account for the influence of the speed of sound. Likewise, comparing indicated airspeed to ground speed or cruise speed to stall speed does not relate to the Mach number directly, since they involve different aspects of flight performance that do not include the fundamental relationship of airspeed to the speed of sound.