Page "Mach number" Paragraph 12

Since the speed of sound increases as the temperature increases, the actual speed of an object traveling at Mach 1 will depend on the fluid temperature around it.

Mach number is useful because the fluid behaves in a similar way at the same Mach number.

So, an aircraft traveling at Mach 1 at 20 ° C or 68 ° F, at sea level, will experience shock waves in much the same manner as when it is traveling at Mach 1 at 11, 000 m ( 36, 000 ft ) at-50 ° C or-58F, even though it is traveling at only 86 % of its speed at higher temperature like 20 ° C or 68 ° F.