Critical Mach Number has nothing (directly) to do with altitude.
Mach Number has everything to do Temperature,
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It can be calculated at any altitude - including negative ones - eg under water.
Mach number is dependent on a number of factors. Factors such as air density or altitude will change the Mach number. At approximatively 45 000 ft Mach 0.85 =289.2361 m/s = 1041.25 km/h = 647.0028 mph
It depends on altitude and humidity Mach I is about 1,225 kilometers per hour.
Ernst Mach .
The Mach number for the speed of an object travelling through a particular medium is the ratio of the object's speed relative to the speed of sound through the same medium. The speed of sound through any medium depends on the nature and density of the medium and, as a result, so does the Mach number. In air, the Mach ratio is mostly dependent on temperature and humidity, less so on altitude. Generally, Mach 1 (in air, at standard sea level conditions, and at 15 deg C) is approx 760 mph. So Mach 1.8 would be approx 1370 mph.