I know I'm pissing into the wind here, but here's a neat little graph







This quantifies the air pressure gradient as you go up in altitude.

But I know what you're thinking. How could there be air pressure at all if there's not some sort of container wall or something against which to pressurize.

I've got that problem all solved for you.

See that thick black line at 30 km? THAT'S the wall against which the air is pressurized. Eureka!