I did not know that mach varied with altitude. Nice impromptu lesson there. Thanks.
de Nada.
However Google is my friend. I knew it dropped with altitude, but I thought it both a temperature and pressure effect.
It's probably not true that temperature is constant about 37,000 feet, even in the standard atmosphere. But I suspect it's constant up to any altitude an air breathing engine would function at. After all, it has to drop to effectively zero at some point, since sound does not propagate in the vacuum of space.