'cloudburst'
<pedantic bump>
The Mach number is the speed of the object divided by the LOCAL speed of sound.
Whilst the speed of sound varies with temperature (and therefore altitude), Mach 1 is always simply the local speed of sound, so to qualify it with an absolute altitude as you have is inaccurate.
</pedantic bump>
:-)
CB
MB1.
"This is a LOCAL speed of sound for LOCAL people...There's nothing for you here!
A stunning example of a Wal Mr Mickey!