Buffet Boundary
 

I'm hoping someone can join the dots on this one please ....

I understand that maximum lift coefficient decreases with altitude due to reducing Reynolds number. I also appreciate that the buffet boundary is usually specified as a single line of CL -v- Mach.

Why does the buffet boundary not also vary with altitude, as maximum lift coefficient does, so you'd also expect the buffet CL to as well?

Thanks in advance.

I'm not sure the Re effect on CLmax is particularly significant. CLmax does decrease with altitude, but mainly because your Mach no. is increasing for a given EAS/IAS, and you start to get compressibility effects

Understood; however above the crossover altitude you're flying a constant Mach number, so then your Mach number is not increasing with altitude.

Of course, but I'm not sure this part:

(at constant mach) is true to any meaningful degree for a real life situation. There is a dependancy of CLmax on Re, but the difference between say Re 5x10^6 and Re 7x10^6 is pretty minimal.

The useful number from an operational perspective is the buffet margin, and it may well be that small changes due to altitude are neglected so that it fits nicely on a 2D graph (pure speculation!)