OF,
Mach number depends on temperature. For example, if you're flying along at Mach 0.98 and you pass through an area of much lower temperature, you can easily exceed Mach 1. Think of Mach number as an indicated measurement... this is valid at any altitude, i.e, flying at Mach 1 at FL360 @ -57 degrees Celcius would also be Mach 1 at 100ft AGL over the North Pole at
-57 degrees Celcius, but only, (for example) .65 Mach at 100ft AGL in the Sahara (+50 degrees Celcius).
In other words:
Low temp: Higher Mach number,
High temp: Lower Mach number.
Clear as mud?
Nial