Questions on high altitude pressures

Not nearly as helpful as some of the other posts, but to give you a
sense of how things change, the pressure at 18,000 feet is about half
of that on the surface (30 inches of Hg on the surface, 15 at 18,000
feet). It's halved again at twice that altitude (7.5 inches at
36,000) and halved again at twice that altitude (3.7 inches at 72,000
feet).




On Nov 24, 1:40*am, es330td wrote:
At low altitudes the rule of thumb is that a one thousand foot
increase in altitude results in a one inch of mercury decrease in
pressure; at least that is what happens when I turn the little knob on
my altimeter. *Since commercial planes flight well into the FL300+
range, clearly that rule cannot hold at altitude as a plane taking off
at 29.92 inches would find itself in a vacuum at 29,900 feet above sea
level. *Can someone with experience of high altitude flight expand on
my understanding of pressure variation?

Also, since the altimeter in the C182 I fly appears to incorporate
that rule of thumb, is it accurate at say, FL120, or is another kind
of altimeter needed for planes flying up there?

TIA