Jose wrote:
It is so useless we don't put it on the ATIS anymore


It's useless because pilots don't know what the number means, or what to
do about it. Pilots aren't told what it means because it's not much
used. It's not much used because...

So... what do the numbers correspond to? That is, if I hear a mu of 33,
what can I expect?

I have no idea. We were told the industry and the FAA got together and
came up with this system and the pilots that wanted the information
would know what the numbers mean. So far nobody ever asks for the mu.